The decompression of error-strings is only done if the string is accessed
via printing or via er.args. Tests are added for this feature to ensure
the decompression works.
Most types are in rodata/ROM, and mp_obj_base_t.type is a constant pointer,
so enforce this const-ness throughout the code base. If a type ever needs
to be modified (eg a user type) then a simple cast can be used.
