Refactor representation of bounds to separate out BuiltinBounds into
its own type. Use a bitset to represent built-in bounds. There
are several places in the language where only builtin bounds (aka kinds)
will be accepted, e.g. on closures, destructor type parameters perhaps,
and on trait types.
rustc: Add EnumSet tests
auto merge of #6336 : nikomatsakis/rust/issue-6308-closure-bounds, r=nikomatsakis

Use a bitset to represent built-in bounds. There are several places in the language where only builtin bounds (aka kinds) will be accepted, e.g. on closures, destructor type parameters perhaps, and on trait types.

r? @brson