WIP
Merge branch 'reb' of github.com:dotty-staging/dotty into reb
Make SubstOnlyTreeMaker maintain type-safety.

Implementation of pattern
sel match {
 x => <body>
}
used to replace references to x by references to sel in <body>.
This is not type correct and it also requires additional traversal of
body.
Now SubstOnlyTreeMaker generates a `x = sel` statement.