Groveller feature proposal: include-local

[Chris Bagley]

Aim:
Add include-local to the groveller.

Behaviour:
- uses #include "name.h" rather than #include <name.h>
- expects the header file to be local to the wrapper/groveler
specification file. This file is copied to the cache dir (with the
generated c file) so it can be used in the build.

Reason:
This allows us to wrap single header libraries simply by making a
wrapper spec containing

> (include-local "some_header_lib.h")

I'd love to hear some thoughts on this. I'm happy to get this done if
it's approved.

Regards
Chris