Pattern-matching spec out of date

[gafter]

The checked-in pattern matching spec is fairly out-of-date, and should be updated per recent discussion and implementation. Specifically,

• The syntactic disambiguation that was added to support patterns in the is-expression needs to be described.
• The use of a wildcard in a type pattern should be described.
• There is an ambiguity between a positional pattern ( 1 ) and a constant pattern with a parenthesized expression. That needs to be resolved, and probably requires some design work.
• The section on the scope of pattern variables should refer to the C# 7 specification.
• The section on a user-defined operator is should be rewritten to refer to a Deconstruct method.
• The spec doesn't describe subsumption as it applies to the switch statement (intuitively, that a case that cannot handle something already handled by a previous case is an error).
• The match-expression requires some design work.
• The throw-expression can be removed (it was added in C# 7)
• I believe we decided to remove the "destructuring assignment" statement.
• It isn't clear if we want a shorthand for a one-arm pattern-matching expression.

Also, some sections of the spec template are missing, such as links to design notes.