[compiler.h][fs] Remove DISABLE_THREAD_ANNOTATIONS from compiler.h

This mechanism is used by some filesystem code to conditionally
disable thread annotations. This is done for the sake of also building
code on the host.

This logic is specific to the filesystem code, and should not be part
of the public Zircon ABI. As such, this CL moves the conditionals into
the filesystem code itself.

Test: built with all compilers; CQ for mac; manually broke threading
analysis to verify that clang is still running the instrumentation.

Change-Id: I9c6453198c637fd03c1238d1c9790cb69ba11b6a

[digest] Switch from cryptolib to uboringssl

This CL switches the SHA-256 implementation for ulib/digest to use
uboringssl, which includes optimized assembly and hardware acceleration
to provide better message digest performance.

Change-Id: Id3dedbc8232b4f4491b19fe46ef9e1415eea7358

[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[zircon][mxtl->fbl] Rename mxtl to fbl

Change-Id: Ie21b6498e1bfb0a7fa0315e40b9e5c3ee78646be

[mxtl][mxalloc] Redirect all clients of mxalloc to mxtl.

Change-Id: Ib6d88937212ab23612f59add8bd028862308ead5

[digest] Rename ulib/merkle to ulib/digest.

The merkle ulib is really about digests, which may or may not be
Merkle-Damgard constructions in the future. As such, this CL performs
several renames in order to make things clearer.

Change-Id: I81377b7afa390e9987ad18f19524e8e3aedd1e69