Avoid cloning key on hotpath

[howardjohn]

                       fastest       │ slowest       │ median        │ mean          │ samples │ iters
                     │               │               │               │         │
      ╰─ old           75.1 ns       │ 87.45 ns      │ 77.06 ns      │ 77.48 ns      │ 100     │ 1000000
      ╰─ new           59.7 ns       │ 74.89 ns      │ 60.89 ns      │ 61.55 ns      │ 100     │ 1000000

This is a breaking change to SelectExpr but a minor one. The member function is private.