cmd/age: touch up help text

age: clean up examples of hardcoded data

age: move package from filippo.io/age/age to filippo.io/age 🤦‍♂️

age,agessh,armor: unleash public API 💥🦑

cmd/age-keygen: print the public key to standard error

internal/age: add a label to the scrypt salt

This way every recipient is labeled with the version.

-----BEGIN AGE ENCRYPTED FILE-----
YWdlLWVuY3J5cHRpb24ub3JnL3YxCi0+IHNjcnlwdCBSbmw2ellyQ1VFK25rVkwx
TkF0SklnIDE4CjlZL2RKb2FOcjFrM0MwSVZqS1BzMUFLeVF5Y1RaMEwvQlRLMWwv
Q0xJbEEKLS0tIEJ1STZCbFh5Vjdsam5nSEFGTVZUY1BxcVVIek04ZUVrOGR4L3ph
NkYzS28KucY25ejFefMDMtKvsAEofDQLsYF41NPrSPITpoxuVWMMZ1ldm+lDh09q
RzCIZAhLN8jaqdeVdCEutqiniJ/9qv4=
-----END AGE ENCRYPTED FILE-----

Password: lies