Code quality in SHA2 streaming: could share more code between finish_224 and finish_256

[msprotz]

Right now, Hacl_SHA2_Streaming.c has two nearly identical functions for Streaming_sha2_224_finish, and Streaming_sha2_256_finish... these two functions call update_224/update_256 (these are identical, we have a proof), update_last_224/update_last_256 (same deal), then finish_224/finish_256.

finish_224 can be expressed in terms of finish_256 followed by a truncation; or worded differently, one could rewrite Streaming_sha2_224_finish to call Streaming_sha2_256_finish then truncate the result, which would save one code size in that file.

(Same deal with 384/512).