Cleaned/reworked bd/caching layer

We really had ~2 duplicate bd layers for a bit there.

This also involved a sort of rewrite of these low-level functions to see
if there were simplifications that could be made.

A couple tweaks:

- Added small low-level lfsr_bd_read/prog/erase/sync_ functions to
  only wrap the bd callbacks and apply any relevant asserts.

  These should be the only place we call the bd callbacks to make it
  easy to read/audit/insert hooks in the future.

- Changed pcache flush lazily, rather than eagerly flushing when full.

  This isn't for any real performance reason, it just makes the code
  simpler. It's not like we can shove more data into the pcache once
  full.

  It's _probably_ a good idea to flush eagerly, to avoid delay more work
  until sync, but I couldn't figure out how to make this work cleanly
  without code duplication...

- Deduplicated read pcache overwrites via lfsr_bd_read__.

  This logic is a bit annoying, but we need the pcache to take priority
  whenever we read from disk, which happens when we both fill our
  rcache, and bypass our rcache. Since these code paths go different
  places, another internal function was the only way I could think to
  deduplicate this.

  It may appear that our pcache/rcache prioritization loop will make
  this happen naturally, as it does in lfs_file_read for example, but
  this doesn't quite work as read-alignment requirements may force us to
  read past the pcache... Keep in mind read_size may be > prog_size.

- Dropped LFS_BLOCK_NULL, now using cache.size=0 to indicate a cache is
  unused.

  This avoids a special lfs_block_t value.

- Dropped lfsr_bd_readcksum, we never used this.

  We can always add it back if necessary.

In total, the caching bd prog/read functions now look quite a bit more
like our file read/write functions, so hopefully that's a good thing.

By the virtue of not have ~2 duplicate bd layers, this saves a bit of
code:

           code          stack
  before: 33700           2800
  after:  33560 (-0.4%)   2808 (+0.3%)

tests/test_fwrite.toml

@@ -1033,7 +1033,7 @@ code = '''
             sim[i] = 'a' + (TEST_PRNG(&prng) % 26);
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
     }
@@ -1545,7 +1545,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;
@@ -1686,7 +1686,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;
@@ -1912,7 +1912,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;
@@ -2052,7 +2052,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;
@@ -2211,7 +2211,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;
@@ -2316,7 +2316,7 @@ code = '''
         }
         lfsr_file_write(&lfs, &file, sim, SIZE) => SIZE;
         size = SIZE;
-    } else if (INIT == 2) {
+    } else {
         memset(sim, 0, SIZE);
         lfsr_file_truncate(&lfs, &file, SIZE) => 0;
         size = SIZE;