lib/cli is very similar to generators, so it should follow largely the
same interface. Similar to how generators isn’t exported, we should
also namespace cli by default (plus “cli” is only three characters to
type).

The semantic difference between `encode` and `to` is not apparent.
Users are likely to confuse both functions (which leads to unexpected
error messages about the wrong types). Like in `generators.nix`, all
functions should be prefixed by `to`.

Furthermore, converting to a string depends on the target context. In
this case, it’s a POSIX shell, so we should name it that (compare
`escapeShellArg` in `strings.nix`).

We can later add versions that escape for embedding in e.g. python
scripts or similar.

Mirrors the naming scheme in `generators.nix`, for consistency.

Also rename `key` to `k` and value to `v` to aid readability to the
code structure.

They are cut off after a few decimal places; we cannot in good faith
define a default string representation with that.

Let’s call them by what they are, option names.

`generators.mkValueStringDefault` is a better value string renderer
than plain `toString`.

Also add docs to all options.

lib: improve cli module