The logic for to-integer and to-float conversions now uses exhaustive
case statements, which fixes both an internal issue with unexpected
literal-node types and conversions-to-char not having range checks.

Perform the range check in a single place. In addition, don't transition
unsigned-valued nodes to `nkUIntLit` -- there's no reason to.

One of the issues encountered in the tests is now fixed. The test case
for `1.23456789012345'f32` was already working previously.