Implement string encryption with ASCII armor

[xecrets]

Implement a simple function to encrypt/decrypt a string under a password, outputting the encrypted data as ASCII armored text. It's ca. 300 bytes overhead, but that's not really of any consequence. Possibly we could in the future do a slimmed down version with less overhead. Since it starts with a GUID, we don't need to wrap it with marker lines, but perhaps
we should anyway to make it easy for humans to know what it is.

It's almost the most common question asked in all forums "How do I encrypt a string" - and this would be a good way to once and for all implement some good reference code to illustrate the actual magnitude of the problem.