Cleanup abilities_in_scope and rename to pending_abilities_in_scope

`abilities_in_scope` is a buffer we use to keep track of locally-defined
abilities before we've fully resolved them. We do this because we
canonicalize ability members signatures before we've registered an
ability to the abilities store, and canonicalization of signatures must
report `has` bounds that don't reference abilities.

So, this buffer is more appropriately named `pending_abilities_in_scope`.
There is also no reason to export it, because it is only relevant
during canonicalization of type defs in a module.

compiler/can/src/expr.rs

@@ -31,7 +31,6 @@ pub struct Output {
     pub introduced_variables: IntroducedVariables,
     pub aliases: VecMap<Symbol, Alias>,
     pub non_closures: VecSet<Symbol>,
-    pub abilities_in_scope: Vec<Symbol>,
 }
 
 impl Output {