Updated pattern match documents

[Brian Goetz]

I've updated the documents regarding pattern matching, and uploaded them 
here:

http://cr.openjdk.java.net/~briangoetz/amber/pattern-match.html
http://cr.openjdk.java.net/~briangoetz/amber/pattern-semantics.html

The first document is an update of a previous document (old version 
available here: 
http://cr.openjdk.java.net/~briangoetz/amber/pattern-match_1.html), and 
outlines the general arc of the feature and general motivation.

The second captures the discussions we've had regarding the messy 
details of typing, scoping, nullability, shadowing, etc.  I think we've 
made a lot of progress on these.

We would not implement this all at once; we'd proceed incrementally, 
probably starting with type patterns in `instanceof`, and then 
proceeding to `switch` or to deconstruction patterns.

Please review and comment.