Add the ability to reference a vector iname directly in a conditional.

This is achived via:

1.  A new VectorLiteral class, based off ArrayLiteral, but with the string mapping changed to have enclosing parenthesis rather than curly braces.
2.  A new VectorTypeCast class, based off TypeCast, but explicitly storing both the base numpy dtype of the VectorLiteral (for inferencing) and the stringified vector dtype (e.g., 'int4') for generation
3.  Modification of the condition mapper wrapper in `wrap_in_if` to properly generate the VectorTypeCast

and is tested in test/test_loopy.py::test_explicit_simd_vector_iname_in_conditional