test multiple dtypes, and correct the specification of the vector length in the condition_mapper_wrapper

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

1.  Literal to vector assignment (no explicit conversion should be used)
2.  Non-vector temporary variable to vector assignment (no explicit conversion)
3.  Vector-temporary to vector variable assignment (explicit conversion)

Previously, only vector to vector assignment was tested.