Part of code cleanup, working towards resolving issue #50.

Part of code cleanup, working towards resolving issue #50.

Part of code cleanup, to resolve issue #50.

Part of code cleanup, towards resolving issue #50.

Addressing issue #50, still some way to go yet.

Top-level lib directory is for standard C libraries that we want to
provide our own versions of (for efficiency and stand-alone reasons).
It currently has libm in it for math functions.

Also add atanf and atan2f, which addresses issue #837.

Found this bug by running unix/ tests with DEBUG=1 enabled when
compiling.

Addresses issue #838.

Optimises:
    if () -> if False
    if (x,...) -> if True
    if (a and b) -> if a and b

This way, the native glue code is only compiled if native code is
enabled (which makes complete sense; thanks to Paul Sokolovsky for
the idea).

Should fix issue #834.

Basic native ARM emitter

The heap allocation is now exactly as it was before the "faster gc
alloc" patch, but it's still nearly as fast.  It is fixed by being
careful to always update the "last free block" pointer whenever the heap
changes (eg free or realloc).

Tested on all tests by enabling EXTENSIVE_HEAP_PROFILING in py/gc.c:
old and new allocator have exactly the same behaviour, just the new one
is much faster.

Recent speed up of GC allocation made the GC have a fragmented heap.
This patch restores "original fragmentation behaviour" whilst still
retaining relatively fast allocation.  This patch works because there is
always going to be a single block allocated now and then, which advances
the gc_last_free_atb_index pointer often enough so that the whole heap
doesn't need scanning.

Should address issue #836.

Saves ROM (16 on stmhal, 240 on 64-bit unix) and should be quicker since
there is 1 less branch.

-t/--target is a pip option. Trying to use pip options for different meanings
in pip-micropython may lead to big confusion. That's why the original passed
any extra parameters using environment variables. "All options belong to pip."

With a file with 1 line (and an error on that line), used to show the
line as number 0.  Now shows it correctly as line number 1.

But, when line numbers are disabled, it now prints line number 1 for any
line that has an error (instead of 0 as previously).  This might end up
being confusing, but requires extra RAM and/or hack logic to make it
print something special in the case of no line numbers.

Fix sdcard_power_on to not do anything if the card is already powered on...

Also add -t/--target option to pip-micropython to allowing installing to
the pyboard.

Thanks to turbinenreiter/Sebastian Plamauer for the patch.

Thanks to Dave Hylands for this patch.

Addresses issue #825.

Addresses issue #827.

SysTick IRQ now increases millisecond counter directly (ie without
calling HAL_IncTick).  Provide our own version of HAL_Delay that does a
wfi while waiting.  This more than halves power consumption when running
a loop containing a pyb.delay call.  It used to be like this, but new
version of HAL library regressed this feature.

Also some whitespace cleanup.

I also removed trailing spaces from modpyb.c which affected a couple
of lines technically not part of this patch.

Tested using: https://github.com/dhylands/upy-examples/blob/master/micros_test.py

which eventually fails due to wraparound issues (I could fix the test to compensate
but didn't bother)

Factored irq functions into a separate file.