This type should be able to replace Separated (with a bit of effort).
The hope is that this type is a better representation for table-like
syntax with optional trailing delimiter that will let me remove_map
elements without too much difficulty. This is necessary for desugaring
table constructors.
I noticed that programs like '{} would parse correctly while '{ } would
expect an inner element. This was because the leading space was actually
part of the element parser, which is a violation of the (as of yet
unspoken) rule that parsers should not parse surrounding whitespace.
Because whitespace whas treated differently from everywhere else and
because this implementation was wrong, I decided to reimplement it,
abstracting the concept of things separated by other things with
optional trailing things. I did this in such a way that surrounding
whitespace is not touched.
