std.crypto: add constant-time codecs
Add constant-time hex/base64 codecs designed to process cryptographic
secrets, adapted from libsodium's implementations.
Introduce a `crypto.codecs` namespace for crypto-related encoders and
decoders. Move ASN.1 codecs to this namespace.
This will also naturally accommodate the proposed PEM codecs.
These are currently incorrect according to the gitattributes(5) and
gitignore(5) man pages. However, it seems github ended up treating them
as we intended due to a bug until recently when that bug was fixed.
With Windows line endings, which seems to be the default on Windows, the
zig compiler won't understand std out of the box. This project should
not rely on git's global core.autocrlf setting.
