These will be used for the PKCS#12 code and to replace some of the
crypto/asn1 logic. So far they support the ones implemented by
crypto/asn1, which are Latin-1, UCS-2 (ASN.1 BMPStrings can't go beyond
the BMP), UTF-32 (ASN.1 UniversalString) and UTF-8.

Change-Id: I3d5c0d964cc6f97c3a0a1e352c9dd7d8cc0d87f2
Reviewed-on: https://boringssl-review.googlesource.com/28324
Commit-Queue: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>
Reviewed-by: Adam Langley <agl@google.com>