Change main function to use extern rust instead of C

[morr0ne]

Currently origin requires users of the library to supply a main function with the following definition

extern "C" {
    fn main(argc: c_int, argv: *mut *mut u8, envp: *mut *mut u8) -> c_int;
}

However it is not necessary to use an extern C there since this never crosses any ffi boundary and is always compiled from source. This pr changes this from an extern C to an extern Rust. This allows to omit any extern declarations when writing main since the Rust extern is always implied.

Main function before this change:

#[no_mangle]
extern "C" fn main(argc: i32, argv: *const *const u8) -> i32 {} 

Main function after this change

#[no_mangle] // No mangle is still required since we are still linking from an external library
fn main(argc: i32, argv: *const *const u8) -> i32 {} 

[morr0ne]

Closed in favor of #34