Using `misc/scripts/fix_headers.py` on all Godot files.
Some missing header guards were added, and the header inclusion order
was fixed in the Bullet module.
When doing large string concatenations the default push_back on the
String class can slow down things quite a bit. This is because it
has to constantly reallocate the memory and copy the contents. This
StringBuilder class delays the concatenation until the size of the
resulting string is known.
