serialize: base64: allow LF in addition to CRLF and optimize slightly

It is useful to have configurable newlines in base64 as the standard
leaves that for the implementation to decide.  GNU `base64` apparently
uses LF, which meant in `uutils` we had to manually convert the CRLF to
LF.  This made the program very slow for large inputs.

[breaking-change]
Could you leave this out? The other two reexports were just added to make the initial namespacing transition easier, but we don't want to keep them around forever.