check_mistyped_turbofish_with_multiple_type_params was previously
expecting type arguments between angle brackets, which is not right, as
we can also see const expressions. We now use generic argument parser
instead of type parser.
Test with one, two, and three generic arguments added to check
consistentcy between
1. check_no_chained_comparison: Called after parsing a nested binop
application like `x < A > ...` where angle brackets are interpreted as
binary operators and `A` is an expression.
2. check_mistyped_turbofish_with_multiple_type_params: called by
`parse_full_stmt` when we expect to see a semicolon after parsing an
expression but don't see it.
(In `T2<1, 2>::C;`, the expression is `T2 < 1`)
- Adds optional default values to const generic parameters in the AST
and HIR
- Parses these optional default values
- Adds a `const_generics_defaults` feature gate
