cli, completion: compress some const strings

[ee7]

This uses supersnappy, which is a zero-dependency pure-Nim
implementation of Google's Snappy.

With it, it is straightforward to compress our larger const strings
at compile time and uncompress them at run time. For now, this commit
compresses:

• The configlet --help message
• The completion scripts

supersnappy is less than 500 lines of code, which is why compressing
these strings (which aren't so large) produces a slightly smaller
configlet binary, despite the added dependency.

We may end up adding zippy as a dependency if configlet learns
to make network requests (e.g. for the online configlet lint checks)
so that we can use gzip. But we can't use zippy at compile time, so it
isn't useful for this commit, even though the compression is better.
zippy is also a bigger dependency (about 40 KiB), so it wouldn't
produce a net saving for this commit anyway.

nim-snappy is an alternative Snappy implementation, but it would
add dependencies on faststreams, unittest2, and stew.

Closes: #651

[ErikSchierboom]

LGTM