This is an attempt to clean up the Micro Python API on the pyboard.
Gpio functionality is now in the Pin object, which seems more natural.
Constants for MODE and PULL are now in pyb.Pin.  Names of some
classes have been adjusted to conform to CamelCase.  Other
miscellaneous changes and clean up here and there.

When querying an object that supports the buffer protocol, that object
must now return a typecode (as per binary.[ch]).  This does not have to
be honoured by the caller, but can be useful for determining element
size.

We are not as verbose as CPython, and maybe a bit too cryptic sometimes.

On stmhal, computed gotos make the binary about 1k bigger, but makes it
run faster, and we have the room, so why not.  All tests pass on
pyboard using computed gotos.

Also make consistent use of MP_OBJ_NOT_SUPPORTED and MP_OBJ_NULL.
This helps a lot in debugging and understanding of function API.

mp_obj_t->subscr now does load/store/delete.

With this fix, all tests in tests/basics pass on pyboard.

mem_info() is already pretty hacky, let it be more hacky.

I.e. with its own globals. So, just as for functions, we need to switch
globals when resuming a generator.

This follows pattern already used for objtuple, etc.: objfun.h's content
is not public - each and every piece of code should not have access to it.
It's not private either - with out architecture and implementation language
(C) it doesn't make sense to keep implementation of each object strictly
private and maintain cumbersome accessors. It's "local" - intended to be
used by a small set of "friend" (in C++ terms) objects.

Addresses issue #487.

Any generated headers go in $(BUILD)/genhdr/, and are #included as
'genhdr/xxx.h'.

Merge branch 'relocatable-build-dir' of github.com:lurch/micropython into lurch-relocatable-build-dir

nlr: Add implementation using setjmp/longjmp.

Add 'test' target to unix/Makefile

Allow the uPy used by run-tests to be overridden

Not all functions implemented.  Not enabled on pyboard.

Things get tricky when using the nlr code to catch exceptions.  Need to
ensure that the variables (stack layout) in the exception handler are
the same as in the bit protected by the exception handler.

Prior to this patch there were a few bugs.  1) The constant
mp_const_MemoryError_obj was being preloaded to a specific location on
the stack at the start of the function.  But this location on the stack
was being overwritten in the opcode loop (since it didn't think that
variable would ever be referenced again), and so when an exception
occurred, the variable holding the address of MemoryError was corrupt.
2) The FOR_ITER opcode detection in the exception handler used sp, which
may or may not contain the right value coming out of the main opcode
loop.

With this patch there is a clear separation of variables used in the
opcode loop and in the exception handler (should fix issue (2) above).
Furthermore, nlr_raise is no longer used in the opcode loop.  Instead,
it jumps directly into the exception handler.  This tells the C compiler
more about the possible code flow, and means that it should have the
same stack layout for the exception handler.  This should fix issue (1)
above.  Indeed, the generated (ARM) assembler has been checked explicitly,
and with 'goto exception_handler', the problem with &MemoryError is
fixed.

This may now fix problems with rge-sm, and probably many other subtle
bugs yet to show themselves.  Incidentally, rge-sm now passes on
pyboard (with a reduced range of integration)!

Main lesson: nlr is tricky.  Don't use nlr_push unless you know what you
are doing!  Luckily, it's not used in many places.  Using nlr_raise/jump
is fine.

Tests in basics (which should probably be renamed to core) should not
rely on float, or import any non-built-in files.  This way these tests
can be run when those features are not available.

All test in basics now pass on the pyboard using stmhal port, except for
string-repr which has some issues with character hex printing.

As discussed in #504

As discussed in #504