IPC RPC codegen for generic implementation

[NikVolf]

another cryptic AST code to allow such things:

#[derive(Ipc)]
impl<L: Sized> DBWriter for DB<L> {
    fn write(&self, data: Vec<u8>) -> Result<(), DBError> {
        let mut writes = self.writes.write().unwrap();
        *writes = *writes + data.len() as u64;
        Ok(())
    }
}