Rename `SmallIntMap` → `HybridDirectMap`.

The old name could be misleading: it is a map of small ints, not a small map of
ints. The new name gives a stronger hint about the implementation strategy and
thus about properties.

`Int` in the name should be avoided because keys will be generalized to a type
with a translation to an unsigned integer type.

`Hash` in the name should be avoided because the fact that the fallback is
a hash map is not very interesting, although this not planned to be generalized.

Rename internals: `SmallMap` → `DirectMap`, `LargeMap` → `SlowMap`.

Use `hybrid_direct_internal` instead of `hybrid_direct_map_internal` though,
anticipating a `HybridDirectSet`.

PiperOrigin-RevId: 889668812