Migrate all Makefile references from script/wren.mk to util/wren.mk, update path for wren_to_c_string.py and libuv.py in build rules, and delete the entire project/msvc2013/ directory containing obsolete Visual Studio 2013 project files.
The Wren scripts in this directory get converted to C string literals into files
with a .wren.inc extension. Those are then #included into their respective
.c files so that the interpreter can load them directly without having to do
any file IO.
The script that does this translation is util/wren_to_c_string.py.
When any of the ".wren" files in here are changed, the Makefile automatically updates the generated C headers.