py/objtype: Change type of enum-to-qstr table to uint16_t to save space.

Qstr values fit in 16-bits (and this fact is used elsewhere in the code) so
no need to use more than that for the large lookup tables.  The compiler
will anyway give a warning if the qstr values don't fit in 16 bits.  Saves
around 80 bytes of code space for Thumb2 archs.