This reverts commit cd6668b5.

This reverts commit 3341ab04.

Closes #160

This reverts merge request !297

added variable_tag to MemAccess; GlobalMemAccessCounter tracking variable tags for tagged global variables

Switching to *None* is not ideal, because *None* is already used in
temp_var_type to represent no value.

Hence, this change introduces an "optional" object wrapper to loopy
and uses it to represent the absence of a temp_var_type in an
assignment instruction.

returned loopy.symbolic.CoefficientCollector to initial state, instead using modified coeff collector (that ignores denom in floor div) locally in stats gatherer; also created corresponding test

Improve test of contiguity in get_access_info, and add a test similar to what triggered the failure in pyjac

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.