swift234

[proposals] MutationModel.rst: edits to define formal language semantics and incorporate results of the last meeting

Some people might object to the way I've defined rvalue and lvalue,
but it just seemed to fall out of things.  I started by talking about
writable and read-only data, and then read-only lvalues, but it was
complicated until all those distinctions fell away and left me with
the basic rule: rvalues can't be mutated and lvalues can.

If theoreticians object, we can find another way to phrase it.

Swift SVN r10010