Remove some data-path CoW

[Lukasa]

Motivation

CoW on the datapath is really gnarly, and we'd like to avoid it if at all possible.

Modifications

• Move some methods into mutating funcs on the state enum, which lets the compiler elide a retain/release pair that force a CoW
• Add proper reset code to a function that throws to ensure that the compiler can see that the state is always restored afterward.

Results

Drastically lowered allocation count