This adds support for (basic) brace expansions with the following
syntaxes:
- `{expr?,expr?,expr?,...}` which is directly equivalent to `(expr expr
expr ...)`, with the missing expressions replaced with an empty string
literal.
- `{expr..expr}` which is a new range expansion, with two modes:
- if both expressions are one unicode code point long, the range is
equivalent to the two code points and all code points between the
two (numerically).
- if both expressions are numeric, the range is equivalent to both
numbers, and all numbers between the two.
- otherwise, it is equivalent to `(expr expr)`.
Closes#3832.
