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