Since ImmutableDictionary and ImmutableSortedDictionary are both
implemented via trees; you can think of an ImmutableDictionary as just
being a sorted dictionary where the underlying key is the hash int.
Since our hash is just a hash of a GUID, we can compare by the GUID,
switch to a ImmutableSortedDictionary, and then avoid another sort
when we're computing checksums.

We previously were using ImmutableArrays, so if you had a large number
of documents (say thousands) we'd be creating huge numbers of huge
arrays over and over again. For this pattern, ImmutableList is more
appropriate.

* IDE Changes

* tests

* PR comments

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.

Add MonoDevelopGtk synchronization context as consumption of Roslyn in VS for Mac uses this context.

We don't need the split anymore now that we're only supporting Dev15.

Unfortunately this required the introduction of a NuGet package that
has a namespace "Workspace" to our VS project, which meant I had to
disambiguate Workspace wherever it's used in that layer. I considered
doing an extern alias instead to disambiguate, but applying that to a
ProjectReference isn't supported. That's tracked by NuGet/Home#4989.

added ability to run OOP in 64bit. currently it is not enabled. but it can be enabled explicitly through options or remotely through a/b testing.

a/b testing is not enabled yet.

* Add and implement IScriptEnvironmentService

* Remove unnecessary export

Fixes #17872