Rename env_md{,_ctx}_st to evp_md{,_ctx}_st

Rename env_md{,_ctx}_st to evp_md{,_ctx}_st

As everyone knows (and who doesn't know will immediately guess), EVP is
short for envelope. Most structs backing the public EVP_* types are called
evp_*. For the EVP_MD and EVP_MD_CTX types, someone used env_md_st and
env_md_ctx_st, which, as jsing pointed out, may or may not be related to
a much less obvious abbreviation of envelope. It could also simply have
been for reasons of inconsistency.

Be all that as it may: rename these structs to use the evp_* namespace
to match all the other EVP types, as well as upstream.

ok jsing