Moved ast_parser to new brian2.parsing package, and removed BitAnd and
BitOr

The reason for removing BitAnd and BitOr is that we want them to be
equivalent to and and or in Python, but they have different precedence.
Previously we were hacking this by replacing '&' with 'and' but this is
confusing. Better to just simply use 'and' and 'or' I think.