Exploit some invariants

Consequently, get rid of ApState.

This speeds up the immediate-indexing test substantially:

Old:

benchmarking <*>/ix1000/500000
time                 2.688 μs   (2.607 μs .. 2.798 μs)
                     0.994 R²   (0.988 R² .. 1.000 R²)
mean                 2.632 μs   (2.607 μs .. 2.715 μs)
std dev              129.9 ns   (65.93 ns .. 242.8 ns)
variance introduced by outliers: 64% (severely inflated)

New:

benchmarking <*>/ix1000/500000
time                 1.410 μs   (1.402 μs .. 1.417 μs)
                     1.000 R²   (1.000 R² .. 1.000 R²)
mean                 1.417 μs   (1.411 μs .. 1.425 μs)
std dev              21.45 ns   (16.80 ns .. 31.73 ns)
variance introduced by outliers: 14% (moderately inflated)