First draft of itypes on prototypes

[aaronjeline]

This branch has the rewriter add I-Types to function prototypes that don't have a body

[aaronjeline]

Here's an example of a program it fails for, and the constraint graph generated.

#include <stddef.h>

extern _Unchecked char *strcpy(char * dest, const char * src : itype(_Nt_array_ptr<const char>));

void foo(void) {
        const char *test = "hello";
        char buf[10] = { 0 };
        strcpy(buf, test);
}

[mattmccutchen-cci]

This PR looks like it may have been an attempt to fix #402, so I'm adding a reference.

[kyleheadley]

How does this relate to #691? Is this PR still something we're pursuing?