rta: guard `add_send` with a specific set, instead of overusing `live_methods`.

Eg. a monomorph method could be live, without being subject to try_send.

Signed-off-by: Jean Privat <jean@pryen.org>
rta: add_monomorphic_send mark the mproperty as live

Signed-off-by: Jean Privat <jean@pryen.org>
rta: do not try to remove things from a set in iteration

Instead, register the things to remove then remove them once the
iteration is over,

Signed-off-by: Jean Privat <jean@pryen.org>