Deduplicate parts of write_enum_operators into write_binary_bitwise_op.

[anforowicz]

PTAL?

Just a minor refactoring / no intended behavior changes - this PR:

• Extracts fn write_binary_bitwise_op out of fn write_enum_operators - the only difference between BitAnd / BitOr / BitXor branches were the op used (& or | or ^) in 1) the name of operatorFOO and 2) in actual expression.
• Also names enum_name so that it can be directly used in formatting specifiers (instead of using {} and then passing enm.name.cxx as an arg)