Custom Queue(M) for Mailbox(M)

[ysbaddaden]

Mailboxes no longer inherit from Channel(M) but use a tailored solution for agent mailboxes that's simpler and with less overall allocations (thanks to Syn::Core).

Kept as DRAFT because:

• This is controversial:
  • we might want timeouts (thus select) but maybe the timeouts should happen differently (e.g. sending a message to the mailbox);
  • we might want to replace the mailbox with a custom Channel(T) but there is no guarantee that Mailbox(M) will always map to a Channel(M). We could wrap M to pass Earl specific messages (e.g. timeout),
• The new Queue(M) object doesn't have any test.