Make the contents of byte-wrapping structs public

This allows users of the library to create keys, read digests, etc.
Revert "The Digest buffer should public (fields are private by default now)"

This reverts commit 69eb7b446256bc9ef2cba3305afdff4705bb77c2.

The reason being that all structs fields should be set to public, better to do
that in one commit.
Merge branch 'PeterReid-master' into rust-master
work around new module/fs rules
replace ~[u8] with Vec<u8>
fix tests
update for Eq trait changes
*T -> *const T
fix --test build
crate_id -> crate_name
add Cargo.toml
Merge branch 'ebfe-fix-build-master' into rust-master
Revert "Update to fit Rust's move from slice to Vec."

This reverts commit 406ec7a37466ac9e8a71c80eb075cf11c170cc65.
Merge tag 'rust-0.11' of https://github.com/dnaq/sodiumoxide
Reintroduce crate_id as we need this for correct library naming.