[compiler.h] Migrate many C++ clients of countof to fbl::count_of

The latter is safer in C++, and the former is a namespace violation in
compiler.h that eventually needs to be resolved.

Test: CQ; should be no functional change
Change-Id: I9809b9fe062fd5f07226323035bf094b3f9de798

[util][utf] Start a UTF conversion library.

Start a basic UTF conversion library. Right now, it only converts from UTF-16
to UTF-8, but it can be extended as needs arise. For now, I plan to use it to
convert strings in USB descriptors (which are spec'ed to be UTF16) to UTF8
(which seems to be the current defacto standard for the rest of Fuchsia)

Change-Id: Ie2bcccb82ec00680752a2fac4d937332bd52fc95