What

• Add wrapper over [String: SourceKitRepresentable] -> SourceKittenDictionary
• Cache File.structure.dictionary

Why

• This is one of the most often called and time-consuming function after the regex matching
  
• Speed up mostly
• Swiftlint using readonly dictionaries from SourceKitten this allows to pre-cache all information used in Swiftlint
• Swiftlint accesses substructure key too often, and since it is a computed property, too many objects will be created and removed upon accessing it
• Having own type instead of extensions over Dictionary will also speed up indexer and compiler a bit

Note

This is more of a suggestion. I totally understand that this is a big change.
Some minor stylistic improvements can be done( like namings and moving extensions in and out)
i.e. this one probably should be moved to File.structurekinds
https://github.com/realm/SwiftLint/pull/2922/files#diff-74fa1a3ab98dac161c19949597728f07R9

This solution calculates all substructures of the original dictionary. Technically, this can lead to regression in time in some cases, if substructure is not used in rules.

This is also can lead to some code duplication, in places, where it is still easier to work with [Strings: SourekitRepresentable]

BTW in order to have backward compatible API, we should probably use duplicated methods which still works with [String: SourceKitRepresentable] and will wrap those under the hood.

Please see #2924 and #2929 for the more improvements, based on this solution