cfunct now tests single C function*

Note:
C can't compile when there are two main methods in the files.
So if you want to test a single C function in foo.c,
foo.c cannot contain a main method. Also, you must have
header file foo.h for foo.c for #include

We should think more about how to deal with this... gcc linker/compiler
is super naive and isn't as smart the Java compiler
add return value example in fib.c and update addTester.svel to test fib()