New kind of EmptyTree for indicating exception selector in Try blocks.

Simplifies a lot handling of Try blocks in patmat and in tailrec.
Typer should leave inline exception handlers inline.
Fix TailRec to handle inline exception handlers and non-trivial not-inlined
Fix Erasure.Boxing isBox and isUnbox

Box& unbox methods are defined in companion objects.
Add Patmat to Context
Stop patmat from using selector pos for Try.

It doesn't exist.
Also use symbol with 'ex' name as selector for exceptions
Flag dotty deviation for return type which is part of cake.
Make typing of Try nodes idempotent
Rewrite assignType for Try

After erasure was always wrong(didn't include the type of handler).
Now it's able to use both Closures and desugared Math nodes.
Restructure Try node.

Now Try node always has cases as handlers.
In case handler is an Ident of type Throwable => T
than it's desugared to a CaseDef during parsing.