Remove unnecessary includes. Add includes that improve portability.

See pull request #343.

Also modify mp_get_index to accept:
1. Indices that are or evaluate to a boolean.
2. Slice indices.
Add tests for these two cases.

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.

Ultimately all static strings should be qstr.  This entry in the type
structure is only used for printing error messages (to tell the type of
the bad argument), and printing objects that don't supply a .print method.

Some tools do not support local/static symbols (one example is GNU ld map file).
Exposing all functions will allow to do detailed size comparisons, etc.

Also, added bunch of statics where they were missing, and replaced few identity
functions with global mp_identity().

Please don't submit patches with tab indentation!

Can't decide which is better for string type, char or byte pointer.
Changing to char removes a few casts.  Really need to do proper unicode.

Takes slice object and sequence length and computes subsequence indexes
for case of slice step=1.

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

Addresses Issue #203.

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

Can now have null bytes in strings.  Can define ROM qstrs per port using
qstrdefsport.h

Note that splitting by explicit string is not implemented so far.

Also, support negative indexes.

That at least makes stuff like "{:x}".format(1) to produce not completely
broken output.