[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

[crypto] Don't publish error.h

The interface for retrieving crypto implementation errors is only used
by the library and shouldn't be under include/.

Change-Id: Id131a086f3db734f72e460a576943ee088c3b36f

[fdio] Move headers into lib/fdio/...

Change-Id: Ie8d74e716da913bf6e2672c4acf8cd67b4962b7f

[crypto][fdio][zxcrypt] Improve debug output

This CL improves fdio/debug's xprintf with location info.

Change-Id: Idd05c0439258ed3fe160086bcce05575eddbc38b

[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 HKDF class

This CL adds HKDF, a key derivation function class, as well as some small
helper files. ulib/digest will eventually need to be merged into
crypto::digest, but it is not necessary for this patch.

Change-Id: Iecaf7eb417d807baead7ffd912980b255ab816bc