Eliminate usages of `ImmutableBuilder`

[joshlemer]

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ImmutableBuilder is currently being used as the default builder implementation for:

• immutable.MultiDict
• immutable.SortedMultiDict
• immutable.MultiSet
• immutable.SortedMultiSet

However, it should be possible, for the sake of performance, to build mutating versions of these, which would mostly be delegating to already-implemented mutating builders of Maps in the standard library (i.e. scala.collection.immutable.{MapBuilderImpl, HashMapBuilder})