Now that we have the extern "C++" trick, we can just embed them in the
normal headers. Move the EVP_CIPHER_CTX deleter to cipher.h and, in
doing so, take away a little bit of boilerplate in defining deleters.

Change-Id: I4a4b8d0db5274a3607914d94e76a38996bd611ec
Reviewed-on: https://boringssl-review.googlesource.com/10804
Reviewed-by: Matt Braithwaite <mab@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>