py: Add simple way of looking up constants in compiler.

Working towards trying to support compile-time constants (see discussion
in issue #227), this patch allows the compiler to look inside arbitrary
uPy objects at compile time.  The objects to search are given by the
macro MICROPY_EXTRA_CONSTANTS (so they must be constant/ROM objects),
and the constant folding occures on forms base.attr (both base and attr
must be id's).

It works, but it breaks strict CPython compatibility, since the lookup
will succeed even without importing the namespace.
stmhal: Add stm module, which contains some constants for the MCU.

Also contains raw memory read/write functions, read8, read16, read32,
write8, write16, write32.  Can now do:

stm.write16(stm.GPIOA + stm.GPIO_BSRRL, 1 << 13)

This turns on the red LED.

With the new constant folding, the above constants for the GPIO address
are actually compiled to constants (and the addition done) at compile
time.  For viper code and inline assembler, this optimisation will make
a big difference.  In the inline assembler, using these constants would
not be possible without this constant folding.