esp8266/modonewire: Move low-level 1-wire bus code to modonewire.c.

The reason it was separated is so that the low-level code could be put in
iRAM, for timing reasons.  But:

1. Tests show that it's not necessary to have this code in iRAM for it to
function correctly, and taking it out of iRAM reclaims some of that precious
resource.  Furthermore, even though these functions were in iRAM there were
some functions that it called (eg pin get/set functions) which were not in
iRAM, so partially defeated the purpose of putting the 1-wire code in iRAM.

2. It's easier to reuse this 1-wire code in other ports if it's in a single
file.

3. If it turns out that certain code does need to be in iRAM then one can
use the MP_FASTCODE macro to do that.