Call ImmutableList<T>.AddRange from ConvertAll and FindAll

Replace any remaining calls to ImmutableList<T>+Node.Add and
ImmutableList<T>+Builder.Add from within a loop to calls to the
equivalent AddRange method
Remove import of System.Linq, reference namespace explicitly.
Address feedback on previous commit

Merge branch 'master' into immutablelist-addrange-refactor

Conflicts:
	src/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableList`1.cs
Merge pull request #112 from PatrickMcDonald/immutablelist-addrange-refactor

Call ImmutableList<T>.AddRange from ConvertAll and FindAll