Add `CZSWAP` gate and optimize `stim.PauliString.{after,before}`

[Strilanc]

• Add CZSWAP gate with SWAPCZ alias
• Add specialized do and undo operations to stim::PauliStringRef instead of using generic methods
  • Measured this as being 10x faster at propagating a stabilizer pauli string through a surface code circuit
• Move gate data from src/circuit/ into src/gates/ directory
• Delete now-unused stim::GateVTable (obsoleted by moving to switch statements)

Fixes https://github.com/quantumlib/Stim/issues/683