This splits introduction of bindings into scope
(`apply_pattern_bindings`) apart from manipulation of the pattern's
binding map (`fresh_binding`). By delaying the latter, we can keep
bindings from appearing in-scope in guards.
Since `fresh_binding` is now specifically for manipulating a pattern's
bindings map, this commit also inlines a use of `fresh_binding` that was
only adding to the innermost rib.
30a0ac66db