py: str.split: handle non-default separator.
py: Add option to compiler to specify default code emitter.

Also add command line option to unix port to select emitter.
py: Implement more features in native emitter.

On x64, native emitter now passes 70 of the tests.
objdict: Support creating dict from another dict.
py: Fix dict.copy() and low-level map/set allocation.

Two things: 1) set flags in copy properly; make mp_map_init() not be too
smart and do something with requested alloc size. Policy of using prime
numbers for alloc size is high-level policy which should be applied at
corresponding high levels. Low-level functions should just do what they're
asked to, because they don't have enough context to be smarter than that.
For example, munging with alloc size of course breaks dict copying (as
changing sizes requires rehashing).
objdict: Implement equality operator.

Sure, it's O(n^2).
tests/dict2.py: Add combined test for dict(dict) and dict equality.
tests: Fix few tests which depend on order of elements in dict.

With dict being unordered of course.
unix-cpy: Pass emit opt to mp_compile.
py: Fix str.replace for case when arg 0 or 1 is empty string.
tests: Fix tests with sets to print sorted set.
pip-micropython: Require command verb, but restrict to "install" only.

So, pip-micropython can/should be run as normal pip:

    pip-micropython install micropython-unittest
py: Revert change to allocation policy for mp_set_t.

Seems that this fixes all set tests.
tests: Fix few tests which depend on order of elements in set.
Merge branch 'master' of github.com:micropython/micropython
tests: Fix more tests which depend on order of elements in set.
objset: Fix incorrect workaround against mp_set_init() munging alloc size.

No longer needed after recent change which guarantees that mp_set_init()
will allocate exact number of slots requested.
py: Revert revert for allocation policy of set hash table.
Replace some Makefile commands with variables in py/mkenv.mk
Merge remote-tracking branch 'upstream/master' into makefile-tweaks