feat(core): implement Context::send in the builder-like manner

[nerodono]

Imperfect draft of turning Context::send into chained builder. Adding behavior settings for Context::send could be annoying, we already have functions like unbounded_send, send, send_to, unbounded_send_to, try_send, try_send_to and so on. With turning send into builder, we could achieve having only one function in the Context: Context::send, actions performed on that builder could be divided in two categories

Outcome

Function which finishes building of envelope and sends it, such as

• ctx.send(..).try_() - try send without waiting for available place in the mailbox
• ctx.send(..).unbounded() - send envelope unconditionally
• ctx.send(..).await - wait for place in the destination mailbox and send

Parameters

• .to(..) - destination

Moreover, we could elide request and request_to functions as well, but it requires more thought though, since ctx.send(..).resolve().await looks nice, but ctx.send(..).all().await seems confusing (Maybe unifying send and request interfaces is quite unfortunate?)