Added caching with managed caches at the file level

This adds a fully independent layer between the rest of the filesystem
and the block device. This requires some additionally logic around cache
invalidation and flushing, but removes the need for any higher layer to
consider read/write sizes less than what is supported by the hardware.

Additionally, these caches can be used for possible speed improvements.
This is left up to the user to optimize for their use cases. For very
limited embedded systems with byte-level read/writes, the caches could
be omitted completely, or they could even be the size of a full block
for minimizing storage access.

(A full block may not be the best for speed, consider if only a small
portion of the read block is used, but I'll leave that evaluation as an
exercise for any consumers of this library)

Author: geky