This is a C++ interface for encrypting and decrypting TLS application
data records in-place, wrapping the existing C API in tls_record.cc.

Also add bssl::Span, a non-owning reference to a contiguous array of
elements which can be used as a common interface over contiguous
container types (like std::vector), pointer-length-pairs, arrays, etc.

Change-Id: Iaa2ca4957cde511cb734b997db38f54e103b0d92
Reviewed-on: https://boringssl-review.googlesource.com/18104
Commit-Queue: Martin Kreichgauer <martinkr@google.com>
Reviewed-by: Adam Langley <agl@google.com>