Spools entire output buffer to a blocking stream (chunk by chunk if
needed).

Includes functions to read and write bits and bytes.

I2C reading tested with TSL2561 luminosity sensor.

Changed from using set_sda_scl function to independent set_sda and
set_scl functions.

Tested and working with SSD1306 I2C display.

This is an initial attempt at making a simple C pin API for writing
things like I2C drivers in C.

It handles more cases than mp_binary_get_int.

FIRST_PRIO=2 works but using 0 is hopefully safer, and can anyway be
optimised in the future.

Requires special lexer to access their contents.

Previous to this patch, the "**b" in "a**b" had its own parse node with
just one item (the "b").  Now, the "b" is just the last element of the
power parse-node.  This saves (a tiny bit of) RAM when compiling.

Hopefully these tests run reliably on Travis.

Previous to this patch, all qemu-arm tests were running in the same
session, and global variables could be left over from the previous test.
This patch makes it so that the heap and runtime are reinitialised at
the start of each test.

All tests in basics/ directory can now run and pass using 64-bit unix
port with only a 16k heap (./run-tests --heapsize 16k).  Tests in this
directory should remain small so they can be used for ports with a
small heap.

This allows you to specify the heapsize that unix will use when running
the test suite, eg: ./run-tests --heapsize 16k

Addresses issue #1898.