Code inspection revealed that it did the wrong thing for annotated trees,
looking in the annotation instead of in the argument.

In particular:

 - get rid of envelope, it's too complicated and hides too many errors
 - check that everywhere in parsed trees the position range of a parent
   node contains the position ranges of its children.
 - check that all non-empty trees coming from parser have positions.

The commit contains lots of fixes to make these checks pass.
In particular, it changes the scheme how definitions are positioned.
Previously the position of a definition was the token range of the
name defined by it. That does obviously not work with the parent/children
invariant. Now, the position is the whole definition range, with the
point at the defined name (care is taken to not count backticks).
Namer is changed to still use the token range of defined name as the
position of the symbol.

Now it's annotated first, annotation second.

This is in line with AnnotatedType and in line with the principle
that tree arguments should come in the order they are written. The
reason why the order was swapped before is historical - Scala2 did it
that way.

Check that children of a node have non-overlapping positions and that positions
of successive children are monotonically increasing.

This holds currently except for 3 exceptions:

 - Trees coming from Java as the Java parser also does desugarings which copy trees.
 - Functions coming from wildcard expressions
 - Interpolated strings

We'll see whether we can do something about the latter two.

Arrange its sub-elements so that they appear strictly left to right.

That way, we can check functions for the ordering requirement as well.
We only have to remember that the last parameter of a wildcard function
does not precede its body (because the parameter is in fact part of the body).