Add compiler error for using @intFromPtr on bit aligned pointer

[Hejsil]

Since bit aligned pointers is represented as meta data on the pointer type at compile time, it doesn't make sense to allow @ptrToInt on these pointers, as the address cannot be represented at runtime.

const A = packed struct {
    a: u4,
    b: u4
};

test \"\" {
    var a: A = undefined;
    _ = @ptrToInt(&a.a); // This is ok. a.a is on a byte bound so we can have a runtime address to it
    _ = @ptrToInt(&a.b); // This should be an error. The address this will return is actually the address to ´a.a´
}

[andrewrk]

This makes sense, but there should be a way to get the address, because there is an address associated with these pointers - it's just that you have to do some bit fiddling to get the value out. The address will be pointing to the "host integer" that contains the bits.