Remove some BASH2PY #if[n]defs by introducing globals

[clarity20]

This seems a hopeful goal in light of the many functions that currently carry an extra parameter when BASH2PY is defined, and given the single-threadedness of the project.

[clarity20]

PR #79 is a different approach toward the same goal of reducing the BASH2PY complications. It breaks out a BASH2PY version of parse.y into its own file.