Merge #1724

1724: Add support for generics associated type binding r=philberty a=philberty

This patch set adds support to specify the associated type via the argument
binding syntax. We are still missing general bounds checking so in order to
properly add more test cases for bad types we need to implement that first.
I have also changed a test case to -fsyntax-only as at the time I always get
confused between Generic arguments of foo=i32 and bar: baz only specifies
the associated type and the latter adds a bound. Currently we are lacking a
way to represent adding a bound in the AST and HIR within generic arguments
so I have raised: #1725 and #1726

Fixes #1720

Co-authored-by: Philip Herron <herron.philip@googlemail.com>