Tweak ImmutableDictionary.TryGetValue to improve throughput (dotnet/corefx#35759)

Tweak ImmutableDictionary.TryGetValue to improve throughput (dotnet/corefx#35759)

* Avoid recursive calls in SortedInt32KeyNode.Search

* Compare first key directly in HashBucket

* Seal a few internal types that have virtual overrides or interface implementations

* Avoid extra level of indirection with Search

Now that it's no longer recursive, we can just inline it and build the caller's decisions right into it.

Commit migrated from https://github.com/dotnet/corefx/commit/8c54cd84f98c553b75356c69d2c8f5a4029717bc