py: Add MICROPY_QSTR_BYTES_IN_LEN config option, defaulting to 1.
This new config option sets how many fixed-number-of-bytes to use to store the length of each qstr. Previously this was hard coded to 2, but, as per issue #1056, this is considered overkill since no-one needs identifiers longer than 255 bytes. With this patch the number of bytes for the length is configurable, and defaults to 1 byte. The configuration option filters through to the makeqstrdata.py script. Code size savings going from 2 to 1 byte: - unix x64 down by 592 bytes - stmhal down by 1148 bytes - bare-arm down by 284 bytes Also has RAM savings, and will be slightly more efficient in execution.