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().
__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.
