Fix for #90 - removing Enum.HasFlags

This is a perf improvement of about 500ms for our razor code generation
benchmark on my dev box. That's about .8% of the overall execution time of
this benchmark.

This change will remove a bunch of unnessary allocations from the
parsing/code-generation path, and should improve responsiveness.

For reference Enum.HasFlags performs boxing of the enum value, and then
does a type comparison to see if the types are the same. This is
significantly more costly than a normal bitwise and comparison, and it
results in allocations.