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.
This adds a new utility to intelligently convert Nix records to
command line options to reduce boilerplate for simple use cases and to
also reduce the likelihood of malformed command lines
