Add foreign constants

This is a quick hack to access constants defined in the foreign
realm of the code generator.

As an example: operating systems define constants for their API
in C header files.
To access them one used to have to write a function that returns the
appropriate values. This patch enables accessing the constants directly
in a way similar to the FFI:

%include C "math.h"

c_pi : Float
c_pi = mkForeignPrim (FConst "M_PI" FFloat)

yields the variable c_pi with the correct type, referring to the
constant M_PI in generated C code. This is currently a C Codegen
feature.