scripts: Adopted crc32c lib when available

Jumping from a simple Python implementation to the fully hardware
accelerated crc32c library basically deletes any crc32c related
bottlenecks:

  crc32c.py disk (1MiB) w/  crc32c lib: 0m0.027s
  crc32c.py disk (1MiB) w/o crc32c lib: 0m0.844s

This uses the same try-import trick we use for inotify_simple, so we get
the speed improvement without losing portability.

---

In dbgbmap.py:

  dbgbmap.py w/  crc32c lib:             0m0.273s
  dbgbmap.py w/o crc32c lib:             0m0.697s
  dbgbmap.py w/  crc32c lib --no-ckdata: 0m0.269s
  dbgbmap.py w/o crc32c lib --no-ckdata: 0m0.490s
  dbgbmap.old.py:                        0m0.231s

The bulk of the runtime is still in Rbyd.fetch, but this is now
dominated by leb128 decoding, which makes sense. We do ~twice as many
fetches in the new dbgbmap.py in order to calculate the gcksum (which
we then ignore...).

scripts/dbgbmapd3.py

@@ -20,6 +20,11 @@ import re
 import shlex
 import struct
 
+try:
+    import crc32c as crc32c_lib
+except ModuleNotFoundError:
+    crc32c_lib = None
+
 
 TAG_NULL        = 0x0000    ## 0x0000  v--- ---- ---- ----
 TAG_CONFIG      = 0x0000    ## 0x00tt  v--- ---- -ttt tttt
@@ -190,12 +195,15 @@ def rbydaddr(s):
     return tuple(addr)
 
 def crc32c(data, crc=0):
-    crc ^= 0xffffffff
-    for b in data:
-        crc ^= b
-        for j in range(8):
-            crc = (crc >> 1) ^ ((crc & 1) * 0x82f63b78)
-    return 0xffffffff ^ crc
+    if crc32c_lib is not None:
+        return crc32c_lib.crc32c(data, crc)
+    else:
+        crc ^= 0xffffffff
+        for b in data:
+            crc ^= b
+            for j in range(8):
+                crc = (crc >> 1) ^ ((crc & 1) * 0x82f63b78)
+        return 0xffffffff ^ crc
 
 def pmul(a, b):
     r = 0