tools/mpy-tool.py: Add ability to merge multiple .mpy files into one.

Usage:

    mpy-tool.py -o merged.mpy --merge mod1.mpy mod2.mpy

The constituent .mpy files are executed sequentially when the merged file
is imported, and they all use the same global namespace.

tools/mpy-tool.py

@@ -691,7 +691,10 @@ def read_mpy(filename):
                 raise Exception('native architecture mismatch')
         config.mp_small_int_bits = header[3]
         qstr_win = QStrWindow(qw_size)
-        return read_raw_code(f, qstr_win)
+        rc = read_raw_code(f, qstr_win)
+        rc.mpy_source_file = filename
+        rc.qstr_win_size = qw_size
+        return rc
 
 def dump_mpy(raw_codes):
     for rc in raw_codes:
@@ -789,6 +792,55 @@ def freeze_mpy(base_qstrs, raw_codes):
         print('    &raw_code_%s,' % rc.escaped_name)
     print('};')
 
+def merge_mpy(raw_codes, output_file):
+    assert len(raw_codes) <= 31 # so var-uints all fit in 1 byte
+    merged_mpy = bytearray()
+
+    if len(raw_codes) == 1:
+        with open(raw_codes[0].mpy_source_file, 'rb') as f:
+            merged_mpy.extend(f.read())
+    else:
+        header = bytearray(5)
+        header[0] = ord('M')
+        header[1] = config.MPY_VERSION
+        header[2] = (config.native_arch << 2
+            | config.MICROPY_PY_BUILTINS_STR_UNICODE << 1
+            | config.MICROPY_OPT_CACHE_MAP_LOOKUP_IN_BYTECODE)
+        header[3] = config.mp_small_int_bits
+        header[4] = 32 # qstr_win_size
+        merged_mpy.extend(header)
+
+        bytecode = bytearray()
+        bytecode_len = 6 + len(raw_codes) * 4 + 2
+        bytecode.append(bytecode_len << 2) # kind and length
+        bytecode.append(0b00000000) # signature prelude
+        bytecode.append(0b00001000) # size prelude
+        bytecode.extend(b'\x00\x01') # MP_QSTR_
+        bytecode.extend(b'\x00\x01') # MP_QSTR_
+        for idx in range(len(raw_codes)):
+            bytecode.append(0x32) # MP_BC_MAKE_FUNCTION
+            bytecode.append(idx) # index raw code
+            bytecode.extend(b'\x34\x00') # MP_BC_CALL_FUNCTION, 0 args
+        bytecode.extend(b'\x51\x63') # MP_BC_LOAD_NONE, MP_BC_RETURN_VALUE
+
+        bytecode.append(0) # n_obj
+        bytecode.append(len(raw_codes)) # n_raw_code
+
+        merged_mpy.extend(bytecode)
+
+        for rc in raw_codes:
+            with open(rc.mpy_source_file, 'rb') as f:
+                f.read(4) # skip header
+                read_uint(f) # skip qstr_win_size
+                data = f.read() # read rest of mpy file
+                merged_mpy.extend(data)
+
+    if output_file is None:
+        sys.stdout.buffer.write(merged_mpy)
+    else:
+        with open(output_file, 'wb') as f:
+            f.write(merged_mpy)
+
 def main():
     import argparse
     cmd_parser = argparse.ArgumentParser(description='A tool to work with MicroPython .mpy files.')
@@ -796,12 +848,16 @@ def main():
         help='dump contents of files')
     cmd_parser.add_argument('-f', '--freeze', action='store_true',
         help='freeze files')
+    cmd_parser.add_argument('--merge', action='store_true',
+        help='merge multiple .mpy files into one')
     cmd_parser.add_argument('-q', '--qstr-header',
         help='qstr header file to freeze against')
     cmd_parser.add_argument('-mlongint-impl', choices=['none', 'longlong', 'mpz'], default='mpz',
         help='long-int implementation used by target (default mpz)')
     cmd_parser.add_argument('-mmpz-dig-size', metavar='N', type=int, default=16,
         help='mpz digit size used by target (default 16)')
+    cmd_parser.add_argument('-o', '--output', default=None,
+        help='output file')
     cmd_parser.add_argument('files', nargs='+',
         help='input .mpy files')
     args = cmd_parser.parse_args()
@@ -835,6 +891,8 @@ def main():
         except FreezeError as er:
             print(er, file=sys.stderr)
             sys.exit(1)
+    elif args.merge:
+        merged_mpy = merge_mpy(raw_codes, args.output)
 
 if __name__ == '__main__':
     main()