* Store checksums as a first-class piece of data in our persistence layer.

* Include the format version in the persistence checksum.

* Move symbol tree over to the checksym approach.

* Update sql persistence service to use checksums.

* Update tests.

* Fix wrapper.

* Remove projects.

* Add tests with checksums.

* Add more tests.

* Add test.

* Add comments.

* Simplify.

* Simplify.

* Revert.

* Revert.

* Revert.

* Add comment.

* Add comment.

* Add comment.

* Rev the checksum.

* Add comment.

* Add comment.

* Add comment.

* Add comment.

* Simplify.

* merge

* rename

* No allow checksym creation

* Rename

* cache strings

* simplify

* Remove optional params

* Update src/Workspaces/Core/Portable/FindSymbols/SyntaxTree/SyntaxTreeIndex_Persistence.cs
Co-Authored-By: NAndrew Harper <aharper@breuer.com>

this basically prevent symbol info indexer from creating/calculating same checksum again and again for every documents in the solution.

this should make it to do it only once per project rather than document, and only update it when there is a change in the project.

The default string comparer uses the current culture, which increases the
time taken to sort this collection, even though the result is never observed
by the user.

Don't make the checksum dependent on the document checksum directly.  The document checksum contains values that change across sessions.