Fix gc_realloc to expand in place

* Issue #322

unix: Allow to set heap size using "-X heapsize=N" option.

Addresses issue #327.

Checks for non-default args following default args, and errors out.
Addresses issue #328.

Implement collections.namedtuple

I.e. as replacement of MP_OBJ_IS_TYPE(), which takes into account subclassing.

Implement machine_sqrt using VSQRT

Support building "unix" port for ARM

Implement not, shl and shr in mpz library.  Add function to create mpzs
on the stack, used for memory efficiency when rhs is a small int.
Factor out code to parse base-prefix of number into a dedicated function.

This change allows to build unix version without libreadline installed.

Remove param from python API of ext_register

Minor cleanup.

Don't need to wrap bitfields in their own struct.  Compiler does the
correct thing without it.

Update LED configuration

Print an error if an invalid BOARD is specified