Implement SHA256_{Update,Transform,Final}() directly in sha256.c.

Implement SHA256_{Update,Transform,Final}() directly in sha256.c.

m32_common.h is a typical OpenSSL macro horror show - copy the update,
transform and final functions from md32_common.h, manually expanding the
macros for SHA256. This will allow for further clean up to occur.

No change in generated assembly.

ok beck@ tb@