Fix unquoted list variables.

[rgrinberg]

This is the naive attempt to fix #699. It still needs some fixing to give proper locs in the case when the error occurs, but I'm not so certain we're taking the right approach here. I don't really like that expand and partial_expand have so much duplicate code and the way "multivalued" expansions are handled. I don't have suggestions how to improve this but I will try to improve the expansion code.

[ghost]

I think to be more future proof regarding the other developments, we should implement exactly the semantic described in #701 (comment).

So technically, I think we can get rid of Concat_or_split and select the right behavior based on the context, which is one of:

1. inside a quoted string
2. as part of an atom but not a whole atom, such as x%{x}
3. as a whole atom in a position where exactly one value is expected, such as (with-stdout-to %{x} (...))
4. as a whole atom as argument of a variadic function, such as (run %{x})

In cases 2 and 3, it is an error if the variable doesn't expand to exactly 1 element. In other cases it may expand to any number of elements.

Additionally, in case 1 and 2, the value is always converted to a string, while in case 3 and 4 it retains it's type, i.e. string or path.

[rgrinberg]

Closed in favor #849