This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.

$(TOP) is defined in py/mkenv.mk and should be used to refer to the top
level of this repository.

It's now generic enough to be used by any port.

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.

It provides sys.stdin, sys.stdout, sys.stderr for bare-metal targets based
on mp_hal functions.

The latest fashion is pushing certificate sub-chains, instead of a single
certificate, during TLS handshake. These are pushed via single TLS record
and effectively put minimum size limit on TLS record buffer. Recently,
these commonly grew over 4K, so we have little choice but to adjust.

Helpful when dealing with frozen modules (and whole applications).