Add strToBytes and bytesToStr tubes

[Julian]

It seems likely to be useful to have some tubes that encode and decode strs.

(Mostly because of #67, otherwise it'd be a 1-liner to inline create such a thing)

[wsanchez]

Duplicate of #60?

[glyph]

#60 is about gathering up the output of a fount into a single str, whereas this seems to be about decoding a stream of bytes into a stream of text (and vice versa)