py/make_root_pointers: Add MP_REGISTER_ROOT_POINTER parser/generator.

This adds new compile-time infrastructure to parse source code files for
`MP_REGISTER_ROOT_POINTER()` and generates a new `root_pointers.h` header
file containing the collected declarations.  This works the same as the
existing `MP_REGISTER_MODULE()` feature.

Signed-off-by: David Lechner <david@pybricks.com>

py/make_root_pointers.py

@@ -0,0 +1,55 @@
+"""
+This pre-processor parses a single file containing a list of
+MP_REGISTER_ROOT_POINTER(variable declaration) items.
+
+These are used to generate a header with the required entries for
+"struct _mp_state_vm_t" in py/mpstate.h
+"""
+
+from __future__ import print_function
+
+import argparse
+import io
+import re
+
+
+PATTERN = re.compile(r"MP_REGISTER_ROOT_POINTER\((.*?)\);")
+
+
+def find_root_pointer_registrations(filename):
+    """Find any MP_REGISTER_ROOT_POINTER definitions in the provided file.
+
+    :param str filename: path to file to check
+    :return: List[variable_declaration]
+    """
+    with io.open(filename, encoding="utf-8") as c_file_obj:
+        return set(re.findall(PATTERN, c_file_obj.read()))
+
+
+def generate_root_pointer_header(root_pointers):
+    """Generate header with root pointer entries.
+
+    :param List[variable_declaration] root_pointers: root pointer declarations
+    :return: None
+    """
+
+    # Print header file for all external modules.
+    print("// Automatically generated by make_root_pointers.py.")
+    print()
+
+    for item in root_pointers:
+        print(item, end=";")
+        print()
+
+
+def main():
+    parser = argparse.ArgumentParser()
+    parser.add_argument("file", nargs=1, help="file with MP_REGISTER_ROOT_POINTER definitions")
+    args = parser.parse_args()
+
+    root_pointers = find_root_pointer_registrations(args.file[0])
+    generate_root_pointer_header(sorted(root_pointers))
+
+
+if __name__ == "__main__":
+    main()