support for inline C++ header (analog to cgo for go)

[timotheecour]

• NOTE: go has some form of this feature (but of course limited to C: https://golang.org/cmd/cgo/); there's an opportunity to do this much more elegantly
• eg use cases: would simplify writing tests, which wouldn't have to carry around separate headers; makes scripting easier

eg syntax:

module main;

enum header=`
#include <stdio.h>
inline void fun(){printf("hello world\n");}
`;

pragma(modmapFromString, header);

import(C++) _:fun;
void main(){fun;}

how hard would that be to implement?

cc @marler8997
EDIT: maybe HAR could be used here, allowing to use multiple files, eg:

module main;

enum har_header=`
--- util.h
#include <stdio.h>
void fun();
--- util.cpp
void fun(){printf("hello world\n");}
`;
pragma(modmapFromString, har_header);