Move `FieldMap` from a private member class of `FieldHandlerMap` to a public
standalone class called `SmallIntMap`.

Its design is not specific to field handlers, and it does not actually depend
on the `Context...` template parameters of `FieldHandlerMap`.

Generalize it a bit by parameterizing it over:
* key type (`int` in `FieldHandlerMap`)
* expected minimum key (1 in `FieldHandlerMap`)
* maximum array capacity (128 in `FieldHandlerMap`)
* source construct it from (`absl::flat_hash_map<int, Value>`
  in `FieldHandlerMap`)

`SmallIntMap` is a map optimized for keys being small integers. It supports only
lookups, but no incremental building nor iteration.

It stores a part of the map covering some range of keys starting from
`expected_min_key` in an array.

At least `array_capacity` possible keys starting from `expected_min_key` are
suitable for array lookup. If all present keys are suitable, the stored array
can be smaller than `array_capacity`, covering the range to the largest key.
If the map is large, the stored array can be larger than `array_capacity`,
as long as it is at least 25% full.

Optimizations to `SmallIntMap`:
* Move `large_map_` behind a pointer to reduce memory usage in the common case.
* Delay constructing elements of `small_values_` to avoid requiring `Value`
  to be default-constructible, and to make the code initializing them smaller.
* Move the slow path of `Find()` to a separate function to inline less code.
* Add `RIEGELI_ASSUME(_ == nullptr)` to avoid generating deletion code for an
  initial assignment to a `std::unique_ptr`.

PiperOrigin-RevId: 879470235