Added lfs_memlen, replacing lfsr_gdelta_size

This drops the last lfsr_gdelta_* function, which were really just a
bunch of somewhat-quirky mem operations.

Moving this to lfs_util.h also allows users to override it with
hardware-specific tricks, though I think hardware tricks for lfs_memlen
will be quite rare. Reverse-order memory optimizations are pretty
uncommon...

This could also be done with a theoretical memrcchr, if one existed:

  p = memrcchr(buffer, 0, size);
  return (p) ? p - buffer : 0;

But I figured this use case is so niche we might as well just limit it
to c=0 (and avoid questions about memrchr).

No code changes.

lfs_util.h

@@ -473,6 +473,16 @@ static inline void *lfs_memcchr(const void *a, int c, size_t size) {
     return NULL;
 }
 
+// Find the minimum length that includes all non-zero bytes
+static inline size_t lfs_memlen(const void *a, size_t size) {
+    const uint8_t *a_ = a;
+    while (size > 0 && a_[size-1] == 0) {
+        size -= 1;
+    }
+
+    return size;
+}
+
 // Xor n bytes from b into a
 static inline void *lfs_memxor(
         void *restrict a, const void *restrict b, size_t size) {