flat_map: Avoid double-templating Pair

As is, the type is a bit obnoxious to name, because you have to use
either FlatMap<K, V, 1>::Pair<K, V> or FlatMap<K, V, 1>::value_type.

Because FlatMap doesn't have any copy semantics, you are pretty much
required to use constructors that take the expected std::array, should a
class want to decorate this type.

Change-Id: I908fdf7d0f4502932d2c1244e8e8b5dc593267a8
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/102130
Commit-Queue: Anqi Dong <anqid@google.com>
Reviewed-by: Armando Montanez <amontanez@google.com>