I started out by adding a new pointer type for blocks, and I was fully
prepared to compile their AST to bytecode and later call them.
... then I found out that the current interpreter doesn't support
calling blocks at all. So we reuse `Function` to support sources other
than `FunctionDecl`s and classify `BlockPointerType` as `PT_FnPtr`.
This happens for enum types with bool parent types. isBooleanType()
returns false for them however.
The previous version did the same thing by re-classifying the enum
integer type, but that breaks with forward-declared enums.
