Implement easy C externs

[mattisboeckle]

Enables writing extern definitions such as

extern def glfwCreateWindow(width: C::Int, height: C::Int, title: C::String, monitor: C::Ptr, share: GLFWwindow): GLFWwindow = c "glfwCreateWindow"

which has to match a linked c function that has the correct name and types.

In this specific case

GLFWwindow* glfwCreateWindow(int width, int height, const char* title, GLFWmonitor* monitor, GLFWwindow* share);

This will automatically generate a wrapper function for said C function, which can then be called from effekt.
There are some selected types and utility functions in effekt.effekt