Add hkdf functions

We might want to make it general by passing in the hmac function/first-class module of type Hacl.HASH.s instead of just supporting SHA256.

Would like to add property based testing but not needed if this is a blocker for someone.

Added the new instrumentation stanza to get test coverage easily.

I'm using this command to run tests and build docs:

dune build --instrument-with bisect_ppx --force \
  @src/lib_crypto/runtest \
  @src/lib_crypto/doc

bisect-ppx-report html

bisect-ppx-report summary --per-file

Then you can look at the coverage in _coverage/index.html and docs in _build/default/_doc/_html/index.html