Add builtin booleans

[j-hui]

Right now, conditional statements use I32 conditions, and boolean operators like == and <= return I32 results, just like their C counterparts. We should really commit to some kind of builtin Boolean type that avoids this hack.

[j-hui]

Ideally we don't just hard-code Booleans, but support arbitrary algebraic data types (kind of like what Coq does). This should be very doable, but may require additional primitives in IR and desugaring after type-checking.