Addresses issue #295.

Thanks to @pfalcon for the tip!

Each built-in exception is now a type, with base type BaseException.
C exceptions are created by passing a pointer to the exception type to
make an instance of.  When raising an exception from the VM, an
instance is created automatically if an exception type is raised (as
opposed to an exception instance).

Exception matching (RT_BINARY_OP_EXCEPTION_MATCH) is now proper.

Handling of parse error changed to match new exceptions.

mp_const_type renamed to mp_type_type for consistency.

Convert unix open() to such.

Linear table at the moment, to eventually be replaced with a hash table
generated by a preprocessor.

Dynamic table is retained so that builtins can be overridden.

sys.path is not initialized by rt_init(), that's left for platform-specific
startup code. (For example, bare metal port may have some hardcoded defaults,
and let user change sys.path directly; while port for OS with environment
feature can take path from environment). If it's not explicitly initialized,
modules will be imported only from a current directory.

Only works on modules and class instances.

Native emitter has been broken since stack order has changed from
reverse to standard.  This fix gets it partially working.

IS_NOT and NOT_IN are now compiled to IS + NOT and IN + NOT, with a new
special NOT bytecode.

TODO: Decide if we really need separate bytecode for creating functions
with default arguments - we would need same for closures, then there're
keywords arguments too. Having all combinations is a small exponential
explosion, likely we need just 2 cases - simplest (no defaults, no kw),
and full - defaults & kw.

rt_call_function_n_kw did check for integers but not for strings
being called. Added a check so running "a"() won't SIGSEV but
throw an exception.

__bool__() and __len__() are just the same as __neg__() or __invert__(),
and require efficient dispatching implementation (not requiring search/lookup).
type->unary_op() is just the right choice for this short of adding
standalone virtual method(s) to already big mp_obj_type_t structure.

We still have FAST_[0,1,2] byte codes, but they now just access the
fastn array (before they had special local variables).  It's now
simpler, a bit faster, and uses a bit less stack space (on STM at least,
which is most important).

The only reason now to keep FAST_[0,1,2] byte codes is for compressed
byte code size.

LOAD_METHOD bug was: emitbc did not correctly calculate the amount of
stack usage for a LOAD_METHOD operation.

small int bug was: int was being used to pass small ints, when it should
have been machine_int_t.

Addresses Issue #203.

This reuses as much str implementation as possible, from this we
can make them more separate as needed.

STM port crashes without this re-init.  There should not be any state in
the core py/ code that relies on pre-initialised data.