Out-of-bounds read in `Log2Cache::nlog2n()`, `nlog2n_difference()` and `plog2p()`

`Log2Cache::new(n)` fills its three tables with entries for `0..=n`, but the
three accessor methods index those tables with `get_unchecked` on a value the
caller supplies. Nothing bounds the argument and nothing documents a bound, so
any value above the `n` the cache was built with reads past the end of the
vector. `CountType` is `u32`, so the read can land up to `u32::MAX` elements
away, and `plog2p()` returns the value it read, disclosing whatever lies there.

For example, `Log2Cache::new(4)` builds tables holding 5 entries (indices
0,1,2,3,4). Then `cache.nlog2n(60000)` reads index 60000, far outside the
allocation. Under AddressSanitizer this faults with `SEGV`; Miri reports it as
undefined behavior at `src/log2cache.rs:42`.

`nlog2n_difference()` is off by one further: in the affected versions `new(n)`
pushes `n` entries into that table rather than `n + 1`, so even
`nlog2n_difference(n)` is out of bounds.

This can be triggered through safe public APIs — `Log2Cache::new()` and any of
`nlog2n()`, `nlog2n_difference()` and `plog2p()` — with no `unsafe` required
from the caller.