Add bool type corresponding to the llvm i1/ C99 bool

The idea is to find a way to represent the llvm i1 type, since llvm
ExecutionEngine also uses libffi now. It's a single bit, but I thought
I'd try to coerce it from an i8 (char). This proof-of-concept requires
more work, especially in the area of tests.
Unfortunately `libffi` doesn't support passing bools, and C doesn't guarantee that bools are passed or represented as unsigned chars, so `&ffi_type_uichar` here should be `NULL`.