Now that we don't use Base64 in keys, let's just go back to the standard
alphabet. Still in the spirit of reducing weirdness, use the PEM column
count, so we can also reuse the lineWriter for the PEM armor.
See https://groups.google.com/d/msg/age-dev/UAjkvLoCr9I/l4Q1h3OPAgAJ.
Use the BIP173 format, which is whole-word selectable, markup safe, and
case insensitive.
AGE-SECRET-KEY-1FPSHVEFQXYSX5MMFDE6ZCGRTV4JHQGRFWSS8WETVDSSX76TVV4JQU272CR
See https://groups.google.com/d/msg/age-dev/UAjkvLoCr9I/l4Q1h3OPAgAJ.
All bech32 Go packages have funky APIs, internal types, or case
handling, so include a heavily refactored version of the reference
implementation, and the tests from github.com/btcsuite/btcutil/bech32.
No need to tie ourselves to GitHub.
The redirect is not set up yet, but as long as there is a replace in the
go.mod the tool can't be installed with "go get" anyway.
Not using age-tool.com because A) I don't actually like the domain and
B) it should be about the spec not the specific implementation.