base64_on_demand.patch: create tables the first time they are needed.

b85decode() already uses such trick to build _b85dec.

Note: a85decode() doesn't use a precomputed table.

> If people think that these functions are too slow, they should propose an extension of the binascii module, rather than micro-optimizing the Python code.

Yes, a C extension may be even faster. But I'm not interested to work on such enhance, I just want to fix the regression in memory usage right now.