Improvements to Tuples

 - fromArray now takes optional length, useful when mapping a resizable buffer to a tuple.
 - New types: IndexOf, Contains, IndicesWhere, ReverseOnto
 - New methods: filter, indexOfType, containsType, reverseOnto
 - reverse methods stabilized
 - New boolean types useful for comparing labels: Conforms, Contains, Disjoint

Improve organization and doc comments

 - Bring some order and regularity into sequence of methods and types
 - Make doc comments more regular and informative

Author: odersky