Use IndexedScore for faster API doc and exact phrase scoring.

[isoos]

• Generic key for TokenIndex and IndexedScore now allows composite keys.
• The dartdoc API symbols index used String concatenation and split in place of composite keys, this change replaces those with the new key. Besides the key operations, we can also spare the Map creation here too.
• The exact phrase matches also benefits from skipping the Map creation and merging the score values directly into the IndexedScore.
• The topApiPages variable still uses a Map, it will be migrated in a subsequent PR (also the _TextResults.score).