Add BuiltinBounds to closure type: parse and handle subtyping,
but do not integrate with kindck etc (requires a snapshot first)
auto merge of #6457 : nikomatsakis/rust/issue-6308-closure-bounds, r=brson

Add BuiltinBounds to closure type: parse and handle subtyping,
but do not integrate with kindck etc (requires a snapshot first)

r? @brson