Find a way to stack allocate sockets

[yorickpeterse]

Description

#783 introduces the ability to define inline types capable of storing inline, copy and heap types.

Sockets are still heap allocated however. This is because internally the runtime mutates the registered flag atomically, and such changes need to be visible across borrows.

We need to find a way such that we can either get rid of the mutations of this field, while still being able to (de/re)register the socket whenever necessary.

Related work

• Find a way to stack allocate closures #786
• Make enums and tuples stack allocated types #780

[yorickpeterse]

This also requires changes to the deadline API: we rely on setting a deadline field on the socket, which won't work if we define it as inline. I'm not a fan of this API to begin with, so perhaps there's a better alternative worth looking into.