Switch back to py/modmath.c, keeping documentation. Add math.nan, math.inf, math.tau

[dhalbert]

math.nan, math.inf, math.tau are in MicroPython as EXTRA_FEATURES but are not in our shared-bindings/math.

tau is just 2*math.pi.

[dhalbert]

This is more complicated than I realized. #354 copied py/modmath.c into shared-bindings and added documentation. But all the improvements to math from upstream since then have not been incorporated.

I think we should undo #354 and figure out how to point to the existing math documentation. In the process let's also turn on the MICROPY_PY_MATH_CONSTANTS.