Support typed parameters inside unions

[rdeits]

Note: this is my first time working with the internals of MacroTools. I may have done something wrong. I'd appreciate as much review attention as you can spare.

This supports typed parameters inside unions for the @match macro (fixes #36 ) by cleaning up some of the quoting procedures inside types.jl. The one sketchy part is that I call eval() in types.jl to convert :Symbol into the type Symbol. That feels wrong to me, and it may be possible to avoid it.

The ensurequoted method in macro.jl exists so that subtb can return a TypeBind directly instead of an expression that somehow interpolates to a TypeBind, while still working with plain Expr and Symbol inputs.

[MikeInnes]

Good stuff! So you're right that the core issue is the inconsistency in the way unions are emitted (as objects) vs type bindings (as expressions). However, I think the right solution to this is actually to go the other way around and have everything be an expression, rather than everything be an object. That should lead to a simpler change that solves all the issues in one fell swoop, and without the eval hack. Let me know if you need more details.

FWIW, I don't think the ensurequote should be necessary here; quoting a non-expression shouldn't have any effect AFAIK.