py/emitinlinerv32: Reduce the footprint of compiled code.

This commit introduces a few changes aimed at reducing the amount of
space taken by the inline assembler once compiled:

* The register string table uses 2 bytes for each qstr rather than the
  usual 4
* The opcode table uses 2 bytes for each qstr rather than the usual 4
* Opcode masks are not embedded in each opcode entry but looked up via
  an additional smaller table, reducing the number of bytes taken by
  an opcode's masks from 12 to 2 (with a fixed overhead of 24 bytes for
  the the masks themselves stored elsewhere)
* Some error messages had a trailing period, now removed
* Error messages have been parameterised when possible, and the overall
  text length is smaller.

Signed-off-by: Alessandro Gatti <a.gatti@frob.it>

Author: agatti