[crypto] Split Bytes class

This CL breaks out the Secret class from the Bytes class. A number of
paranoid but expensive functions are only needed for security sensitve
secrets, e.g. mandatory_memset in the destructor of keys. This split
allows only the data that needs the paranoia to pay for it.

Change-Id: Ib3fdc23ef7c0f86a6549c639353ac72bb35ebedf

[ulib/crypto] Bytes class improvements

This CL adds a few more helper methods and tweaks to Bytes:
- Copy(const Bytes &) copies from another Bytes object.
- Resize(size_t) is a no-op if the size is unchanged.
- Randomize(0) will skip the resizing step.
- Merge(const Bytes &) copies another Bytes to the end of this one.
- Split(Bytes*) copies to another Bytes from the end of this one.

Change-Id: I9fced1b700285b51e8e8461472cf5ee6dccb5640

[crypto] Add HMAC class

This CL adds an HMAC, a hash-based message authentication code, that can
be used to verify the integrity of data.

Change-Id: I9b2f5e94cefd9de66529b3eb8b41ff1755fab778