Improve lib-crypto namespace

The following discussion from !6302 (merged) should be addressed:

• @raphael-proust started a discussion: (+2 comments)

  The namespace of Tezos_crypto is all over the place. The other libraries that are included in TzPervasives are somewhat better behaved:

  • Stdlib:
    • Has a tezos_stdlib.ml to list the modules explicitly making merlin happier and making it easier for devs to get their head around it.
    • Has modules with somewhat meaningful names (mostly): Bits Bloomer Bounded_heap Circular_buffer Compare FallbackArray FunctionalArray Hash_queue TzHex Lwt_dropbox Lwt_idle_waiter Lwt_pipe Lwt_utils Memory Tag TzEndian TzFilename TzList TzString Utils
  • Tezos_RPC: all modules start with the RPC_ prefix.
  • Tezos_clic: has a single module called Clic
  • Utils.Infix: exports a single operator (--)
  • Tezos_error_monad.Error_monad: ok that one is a mess as well.

  Compare with Tezos_crypto's naked modules such as S, Helpers, etc.

  I think we can improve lib-crypto in general. Ping @hhugo1 @romain.nl.