Replace the internal usage of Cstruct.t by string (#146)

Originally, we used Cstruct.t (bigarray) for interfacing. Instead, we use string now.
The benefit is that allocating a string is cheap, and in line with OCaml's GC.

After some years of stalling, we included benchmarks in bench/speed.ml fot the EC
operations in #192  (sign, verify, generate for EC/EdDSA; and ECDH). The result for
thi change is a factor between 2 and 2.5.

The external API (mirage_crypto_ec.mli) does not change at all. There are various
other cleanups in the code, such as providing a layer to isolate the C calls (which
receive a bytes buffer for the result value, and thus mutate this buffer) to be
immutable.

Co-authored-by: Pierre Alain <pierre.alain@tuta.io>
Co-authored-by: Hannes Mehnert <hannes@mehnert.org>
Co-authored-by: Reynir Björnsson <reynir@reynir.dk>
Reviewed-by: Virgile Robles <virgile.robles@protonmail.ch>
Reviewed-by: Pierre Alain <pierre.alain@tuta.io>

ec/dune

@@ -1,7 +1,7 @@
 (library
  (name mirage_crypto_ec)
  (public_name mirage-crypto-ec)
- (libraries cstruct eqaf.cstruct mirage-crypto mirage-crypto-rng)
+ (libraries cstruct eqaf mirage-crypto mirage-crypto-rng)
  (foreign_stubs
   (language c)
   (names p224_stubs np224_stubs p256_stubs np256_stubs p384_stubs np384_stubs