Firs version of patching that can be invoked by dotty compiler
itself.

Driver should not know that patch functionality exists. Instead, introduce settings
that can introduce their own stateful values.

No more leaking ofMove PatchedFiles in a settings option. Move all
patch classes into a `Rewrites` object.

Map typed to corresponding untyped trees.

Imports are missing afterwards.

Selectors should be defs, not lazy vals.

Some random neg tests from previous experiments.

Revert this commit once scala#1149 is fixed.

Scala2 allows `x _` even if `x` is not a method.
Dotty disallows them. The patch removes the ` _`
in these cases.

Test rewritings that were implemented so far.

Remove println; add docs

Gave overlapping positions in the case of longer lists of children.

`Iterator.sliding(2, 1)` returns a one-element result if the original iterator
contains only one element, which makes it unpleasant to use for our task. Replaced
by a fold.

A constructor

     def this() { ... }

needs to be rewritten to

     def this() = { ... }

not to

     def this(): Unit = { ... }

1. trailing `_`:

    `x _` is rewritten to `(() => x)` not to `x`

2. lazy vals:

Rewrites are done in Typer, not LazyVals. Later on we are too much at risk to
hit synthetically generated lazy vals.