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.

py/makeqstrdata.py

@@ -73,16 +73,27 @@ def do_work(infiles):
                 # add the qstr to the list, with order number to retain original order in file
                 qstrs[ident] = (len(qstrs), ident, qstr)
 
-    # process the qstrs, printing out the generated C header file
+    # get config variables
+    cfg_bytes_len = int(qcfgs['BYTES_IN_LEN'])
+    cfg_max_len = 1 << (8 * cfg_bytes_len)
+
+    # print out the starte of the generated C header file
     print('// This file was automatically generated by makeqstrdata.py')
     print('')
+
     # add NULL qstr with no hash or data
-    print('QDEF(MP_QSTR_NULL, (const byte*)"\\x00\\x00\\x00\\x00" "")')
+    print('QDEF(MP_QSTR_NULL, (const byte*)"\\x00\\x00%s" "")' % ('\\x00' * cfg_bytes_len))
+
+    # go through each qstr and print it out
     for order, ident, qstr in sorted(qstrs.values(), key=lambda x: x[0]):
         qhash = compute_hash(qstr)
         qlen = len(qstr)
         qdata = qstr.replace('"', '\\"')
-        print('QDEF(MP_QSTR_%s, (const byte*)"\\x%02x\\x%02x\\x%02x\\x%02x" "%s")' % (ident, qhash & 0xff, (qhash >> 8) & 0xff, qlen & 0xff, (qlen >> 8) & 0xff, qdata))
+        if qlen >= cfg_max_len:
+            print('qstr is too long:', qstr)
+            assert False
+        qlen_str = ('\\x%02x' * cfg_bytes_len) % tuple(qlen.to_bytes(cfg_bytes_len, 'little'))
+        print('QDEF(MP_QSTR_%s, (const byte*)"\\x%02x\\x%02x%s" "%s")' % (ident, qhash & 0xff, (qhash >> 8) & 0xff, qlen_str, qdata))
 
     return True