Fix typos in XML comments
System.Xml.XmlDocument.Tests formatter changes
System.Xml.XmlDocument.Tests Sanitizer changes
Replacing foo:bar with val1:val2
Restoring TODOs in comments
Merge branch 'xmltypo'
Remove unused using statements
Sort usings.
Merge branch 'remove-unused-usings'
Fix build break due to using being removed that was actually used.
Fix System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() to not generate Comparer wrapper each time
System.Collections.Immutable!ImmutableArray<T>.Builder.Sort() (3 overloads) creates a new Comparer wrapper each time. This can get expensive if you call Sort a lot.

This fix optmizes method Sort() alone among the 3 overloads. Sort() uses the default comparer which through this fix is getting cached into a static field in type Comparer such that the Comparer wrapper is not created each time Sort() is called.
Existing testcase Sort(), covers this case.
Clean up XML code