Fix BufSectored so it doesn't have to write to the first sector every flush.

Track position and dirty-ness in Trailered.

Implement a stream which is both Read and Write and which can transparently compress and decompress
data written to and read from it.

Remove TryCompose?

Move crypto::{encrypt, decrypt} into corresponding {EncrypterExt, DecrypterExt}.

Create an enum to eliminate the use of Block trait objects?

Add a ser_sign_into method to SignerExt which serializes a value into a provided Vec<u8> and returns
a signature over this data. Update BlockStream::flush_integ to use this method.

Convert all sector sizes to u64 for portability.

Create an extension trait for u64 with a method for adding an i64 to it. Use this in
SecretStream::seek, Trailered::seek and SectoredBuf::seek.

Create a struct which digests data written to it before passing it to an underlying Write.

Fix bug where writing to a block that already has a Writecap in its header using the creds of
a different node produces an invalid signature (a signature using the creds of the other node).

Create a struct called WritecapBody to contain the fields of Writecap which go into the signature
calculation so that WritecapSigInput is no longer required.