A C function import syntax

[kitech]

Currently use fn C.foo(), and when call with C.foo()

I proposol a syntax,

@[c_import: foo_inc]
fn foo(int) int

It use no body function feature, not need prefix with C. when call.
Also V can check function parameter type more, because it is a V function.
Also it can import as public/private function, resolve previous C.foo() confuse visibility problem.

Now it can seem as a V function.