Write used references file only if different

[KirillOsenkov]

Since the analyzer runs on every keystroke in the IDE it's important to reduce I/O and first-chance exceptions

Sample exception:

System.IO.IOException: The process cannot access the file 'C:\ReferenceTrimmer\src\Analyzer\obj\Debug\netstandard2.0\_ReferenceTrimmer_UsedReferences.log' because it is being used by another process.
   at void System.IO.File.WriteAllText(string path, string contents)
   at void ReferenceTrimmer.Analyzer.ReferenceTrimmerAnalyzer.WriteUsedReferencesToFile(HashSet<string> usedReferences, string declaredReferencesPath) in C:/ReferenceTrimmer/src/Analyzer/ReferenceTrimmerAnalyzer.cs:line 161
   at void Microsoft.CodeAnalysis.Diagnostics.AnalyzerExecutor.ExecuteCompilationActions(ImmutableArray<CompilationAnalyzerAction> compilationActions, DiagnosticAnalyzer analyzer, CompilationEvent compilationEvent, CancellationToken cancellationToken)+((Action<CompilationAnalysisContext> action, CompilationAnalysisContext context) data) => { } in C:/Roslyn/src/Compilers/Core/Portable/DiagnosticAnalyzer/AnalyzerExecutor.cs:line 332

[KirillOsenkov]

Fixes #89

[KirillOsenkov]

@stan-sz @inthemedium @dfederm

[KirillOsenkov]

Note this also sorts the references for easier diffing and stability (even though I'm pretty sure the compiler would return these in a deterministic order anyway). If these are not sorted, comparing them is hell. We should perhaps also sort the declared references list.