ccall: allow to use a header file

[certik]

Right now, every @ccall always generates a C forward declaration (in the C backend). In addition, we would also like to have a mode, something like:

@ccall(header="some_header.h")
def f(x: i32) -> f64:
    pass

@ccall(header="some_header.h")
def g(x: i32) -> f64:
    pass

# later
e = f(5)
h = g(5)

Which would generate the following code:

#include "some_header.h"

...
e = f(5)
h = g(5)

and it would not forward declare f and it would only include some_header.h once. So in particular, if f is actually a C macro declared in the some_header.h file, it would also just work.

Alternative syntax:

from ltypes import headers

h = headers("some_header.h")
@ccall(h)
def dummy():
    pass

@ccall
def f(x: i32) -> f64:
    pass

# later
e = f(5)

We might need to figure out the best design, but we need this feature.

[certik]

To simplify, one idea might be:

with ltypes.header("some_header.h"):
    def f(x: i32) -> f64:
        pass

    def g(x: i32) -> f64:
        pass

Another idea:

with ltypes.library(headers=["some_header.h"], lib="something"):
    def f(x: i32) -> f64:
        pass

    def g(x: i32) -> f64:
        pass

And this would use the header file with the C backend, and it would use the lib (with the proper prefix and suffix that is platform dependent) with the pure Python mode as well as the LLVM mode.