toPath has confusing semantics and is never necessary; it can always
either just be omitted or replaced by pre-concatenating `/.`. It has
been marked as "!!! obsolete?" for more than 10 years in a C++
comment, hopefully removing it will let us properly deprecate and,
eventually, remove it.
So far, `mkValueString` defaulted to `toString`,
which is a bad match for most configuration file formats,
especially because how booleans are formatted.
This also improves error messages for unsupported types.
Add a test to codify the formatting.
This means the generation of values can now be influenced, even down from e.g.
an INI generator.
Breaks the interface of `mkKeyValueDefault` to match its interface to other
generator functions. It might me sensible to rename `mkKeyValue` and
`mkKeyValueSet` to conform to the `toX`-style of generator functions.
With `builtins.functionArgs` we can get some information if the first argument
is an attrset and whether the contained fields have default values. Encode that
into the pretty-printed lambda.
