Not that these functions can actually fail. The only codepaths that do so are
user errors.

Change-Id: I9fcbd402ab6574b5423ae22b462a0e1192ef01d7
Reviewed-on: https://boringssl-review.googlesource.com/1900
Reviewed-by: Adam Langley <agl@google.com>

Initial fork from f2d678e6e8 (1.0.2 beta).

(This change contains substantial changes from the original and
effectively starts a new history.)