mescc: Tinycc support: struct.struct.

mescc: Tinycc support: struct.struct.

* module/language/c99/compiler.mes (field:name):
  (field:pointer):
  (field:size):
  (field:type):
  (field-field):
  (field-offset):
  (struct-field): Support struct.struct.
* scaffold/tests/7i-struct-struct.c: Test it.
* make.scm (add-scaffold-test): Build it.