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

Add EXTI support

Fix floating point printing of negative numbers in the range (0, -1.0)

Also change int -> machine_int_t where appropriate.

With this option selected, only HID on its own works, not VCP+HID.

Revamp usrsw to use new exti code.
Add user switch callback function.

Stack layout is different when -g used, so need to handle the pendsv
jump differently.  Addresses Issue #315.

Add Configurable LFN support to FatFS

Some functionality is still missing (eg and, or, bit shift), and some
things are buggy (eg subtract).