Improve a number of numeric function types.

Closes PR13468.
Fix optimizations for complex unboxing.

Omitted parts were not used properly.
Fix optimization of unary complex - and /.
Make tc-let do unions correctly.
Add a simple-Union and Bottom, and have code use them.

Requires splitting out the match expanders from base-abbrev.
Make lists with minimum lengths be a subtype of sequences.