Pattern matching and match expressions

Patterns in Q# can be extended to support UDTs, literals, etc. Match expressions would be useful for branching on an integer or string value, or a value of a sum type (#483). See microsoft/qsharp-language#81.

Start by reading the Q# language discussion in microsoft/qsharp-language#81 and the related sum-type issue #483. Define the scope for extending patterns to UDTs and literals and for match expressions over integer, string, or sum-type values; done requires an agreed design and implementation plan.

Written by the indexing model from the issue text.