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.

TestAssets/TestProjects/TestLibraryWithAnalyzer/project.json

@@ -1,8 +1,5 @@
 {
   "version": "1.0.0-*",
-  "compilationOptions": {
-    "emitEntryPoint": true
-  },
   "dependencies": {
     "NETStandard.Library": "1.5.0-rc2-24022",
     "System.Runtime.Analyzers": {