Changed most references to crc/csum -> cksum

The reason for this is to move away from the idea that littlefs is
strictly bound to CRCs and make the code more welcoming to other
checksum types, such as SHA256, etc.

Of course, changing the name doesn't really do anything. littlefs
actually _is_ strictly bound to CRCs in a couple ways that other
filesystems aren't. These would need to have workarounds for other
checksum types:

- We leverage the parity-preserving nature of (some) CRCs to not have
  to also calculate the parity of metadata in rbyd commits.

- We leverage the linearity of CRCs to retroactively flip the
  perturb bit in the cksum tag without needing to recalculate the
  checksum. Though the fact we need to do this is because of how we
  use parity above, so this may just not be needed for non-CRC
  checksums.

- The plans for global-CRCs (not yet implemented) rely heavily on the
  mathematical properties of CRC polynomials. This doesn't mean
  global-CRCs can't work with other checksums, you would just need to
  find a different type of polynomial.

scripts/dbgmtree.py

@@ -29,8 +29,8 @@ TAG_DID         = 0x0307
 TAG_UATTR       = 0x0400
 TAG_SATTR       = 0x0500
 TAG_ALT         = 0x4000
-TAG_CRC         = 0x2000
-TAG_FCRC        = 0x2100
+TAG_CKSUM       = 0x2000
+TAG_FCKSUM      = 0x2100
 
 
 # parse some rbyd addr encodings
@@ -106,16 +106,16 @@ def frommdir(data):
     return blocks
 
 def frombtree(data):
-    crc = fromle32(data)
+    cksum = fromle32(data)
     w, d1 = fromleb128(data[4:])
     trunk, d2 = fromleb128(data[4+d1:])
     block, d3 = fromleb128(data[4+d1+d2:])
-    return w, trunk, block, crc
+    return w, trunk, block, cksum
 
 def popc(x):
     return bin(x).count('1')
 
-def xxd(data, width=16, crc=False):
+def xxd(data, width=16):
     for i in range(0, len(data), width):
         yield '%-*s %-*s' % (
             3*width,
@@ -175,13 +175,13 @@ def tagrepr(tag, w, size, off=None):
             tag & 0xff,
             ' w%d' % w if w else '',
             size)
-    elif (tag & 0xff00) == TAG_CRC:
-        return 'crc%x%s %d' % (
+    elif (tag & 0xff00) == TAG_CKSUM:
+        return 'cksum%x%s %d' % (
             1 if tag & 0x1 else 0,
             ' 0x%x' % w if w > 0 else '',
             size)
-    elif tag == TAG_FCRC:
-        return 'fcrc%s %d' % (
+    elif tag == TAG_FCKSUM:
+        return 'fcksum%s %d' % (
             ' 0x%x' % w if w > 0 else '',
             size)
     elif tag & 0x4000:
@@ -257,8 +257,8 @@ class Rbyd:
 
         # fetch the rbyd
         rev = fromle32(data[0:4])
-        crc = 0
-        crc_ = crc32c(data[0:4])
+        cksum = 0
+        cksum_ = crc32c(data[0:4])
         off = 0
         j_ = 4
         trunk_ = 0
@@ -270,25 +270,25 @@ class Rbyd:
         trunkoff = None
         while j_ < len(data) and (not trunk or off <= trunk):
             v, tag, w, size, d = fromtag(data[j_:])
-            if v != (popc(crc_) & 1):
+            if v != (popc(cksum_) & 1):
                 break
-            crc_ = crc32c(data[j_:j_+d], crc_)
+            cksum_ = crc32c(data[j_:j_+d], cksum_)
             j_ += d
             if not tag & 0x4000 and j_ + size > len(data):
                 break
 
-            # take care of crcs
+            # take care of cksums
             if not tag & 0x4000:
-                if (tag & 0xff00) != TAG_CRC:
-                    crc_ = crc32c(data[j_:j_+size], crc_)
-                # found a crc?
+                if (tag & 0xff00) != TAG_CKSUM:
+                    cksum_ = crc32c(data[j_:j_+size], cksum_)
+                # found a cksum?
                 else:
-                    crc__ = fromle32(data[j_:j_+4])
-                    if crc_ != crc__:
+                    cksum__ = fromle32(data[j_:j_+4])
+                    if cksum_ != cksum__:
                         break
                     # commit what we have
                     off = trunkoff if trunkoff else j_ + size
-                    crc = crc_
+                    cksum = cksum_
                     trunk_ = trunk__
                     weight = weight_
 
@@ -531,7 +531,7 @@ class Rbyd:
             if branch is not None and (
                     not depth or depth_ < depth):
                 tag, j, d, data = branch
-                w_, trunk, block, crc = frombtree(data)
+                w_, trunk, block, cksum = frombtree(data)
                 rbyd = Rbyd.fetch(f, block_size, block, trunk)
 
                 # corrupted? bail here so we can keep traversing the tree
@@ -805,7 +805,7 @@ def main(disk, mroots=None, *,
         if not args.get('depth') or mdepth < args.get('depth'):
             done, rid, tag, w, j, d, data, _ = mroot.lookup(-1, TAG_MTREE)
             if not done and rid == -1 and tag == TAG_MTREE:
-                w, trunk, block, crc = frombtree(data)
+                w, trunk, block, cksum = frombtree(data)
                 mtree = Rbyd.fetch(f, block_size, block, trunk)
 
                 mweight = w
@@ -1474,7 +1474,7 @@ def main(disk, mroots=None, *,
         if not args.get('depth') or mdepth < args.get('depth'):
             done, rid, tag, w, j, d, data, _ = mroot.lookup(-1, TAG_MTREE)
             if not done and rid == -1 and tag == TAG_MTREE:
-                w, trunk, block, crc = frombtree(data)
+                w, trunk, block, cksum = frombtree(data)
                 mtree = Rbyd.fetch(f, block_size, block, trunk)
 
                 # traverse entries