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.

src/dotnet/Program.cs

@@ -111,7 +111,7 @@ namespace Microsoft.DotNet.Cli
                 command = "help";
             }
 
-            int exitCode;            
+            int exitCode;
             Func<string[], int> builtIn;
             if (s_builtIns.TryGetValue(command, out builtIn))
             {
@@ -175,16 +175,16 @@ namespace Microsoft.DotNet.Cli
         {
             return (shortName != null && candidate.Equals("-" + shortName)) || (longName != null && candidate.Equals("--" + longName));
         }
-        
+
         private static string GetCommitSha()
         {
             var versionFile = DotnetFiles.VersionFile;
-            
+
             if (File.Exists(versionFile))
             {
                 return File.ReadLines(versionFile).FirstOrDefault()?.Substring(0, 10);
             }
-            
+
             return null;
         }
     }