Revert "Remove all transmute calls in generated code"

[EPashkin]

Part of #901

This reverts commit 5ca26d9.

Comment from #903:

Unfortunately, it wasn't a successful experiment... A solution is still possible without transmute:

fn foo() {
    println!("bla");
}

fn bar(i: u32) {
    println!("bar");
}

fn main() {
    unsafe {
        let b = bar as fn(u32);
        let b = &b;
        let b = b as *const fn(u32) as *const fn();
        (*b)();
    }
}

But transmute seems to be the simplest solution. So let's go back to it.

[sdroege]

For completeness, can we do

Suggested change

	"\tSome(transmute({}::<{}F> as usize)), Box_::into_raw(f))",
	"\tSome(transmute::<_, unsafe extern "C" fn()>({}::<{}F> as *const ())), Box_::into_raw(f))",

That's less likely to fall apart and closer to the unsafe code guidelines.

[EPashkin]

Ok, but without second ::

[sdroege]

Indeed, updated comment

[EPashkin]

Some(transmute::<_, unsafe extern "C" fn()>(
                    activate_trampoline::<Self, F> as *const (),
                )),

[sdroege]

Looks correct

[EPashkin]

Only problem of this change that we need change manual code again instead of just reverting commit :(

[sdroege]

Need to do that anyway, there were some other changes in the manual code that should be kept

[sdroege]

@EPashkin Can you update?

[EPashkin]

Oops, forgot to push.
Done

[sdroege]

Thanks, let's get this in then and then fix up everything else :)