Memory usage improvements in build (#2626)

* Use a WorkspaceContext in dotnet-build to cache project data across
multiple compilations in a single build action
* Dramatically reduce string and object duplication by introducing a
"Symbol Table" that shares instances of NuGetVersion, NuGetFramework,
VersionRange and string across multiple lock-file parses

Test Results:
* Testing was done by compiling Microsoft.AspNetCore.Mvc (and it's
dependencies) and taking memory snapshots after each compilation in
dotMemory
* We used to allocate ~3MB and deallocate ~2.5MB on EACH compilation in
a single build action. This has been reduced to ~120KB
allocated/deallocated
* After introducing WorkspaceContext, total memory usage spiked from 6MB
across the whole build action to about 13MB, introducing the symbol
table dropped it back to about 5-6MB.

test/dotnet-test.UnitTests/project.json

@@ -28,7 +28,8 @@
     }
   },
   "content": [
-    "../../TestAssets/TestProjects/ProjectWithTests/project.json"
+    "../../TestAssets/TestProjects/ProjectWithTests/project.json",
+    "../../TestAssets/TestProjects/ProjectWithTests/project.lock.json"
   ],
   "testRunner": "xunit"
 }