The 1-wire bus is defined with fixed timings so there should be no need to
change them dynamically at runtime. Making the timings fixed saves about
270 bytes of code and 20 bytes of RAM.
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.
Previously they used historical "pyb" affix causing confusion and
inconsistency (there's no "pyb" module in modern ports; but people
took esp8266 port as an example, and "pyb" naming kept proliferating,
while other people complained that source structure is not clear).
