So far pathTo(span) would only guarantee to return the path
to the closest tree enclosing `span` if trees were non-overlapping.
This is property does not hold for typed trees, yet `pathTo` is called
from `Interactive` on such typed trees.

This commit makes `pathTo` work correctly also for overlapping trees.

With given coming last, some of the no-overlaps and in-order position
checks have to be weakened. But with the more generalized pathTo
implementation, non-overlapping and in-order is not really needed anymore.

They printed as normal arguments before.

The previous syntax was unparseable without major contortions in the Parser.

Can do it now that we have a full bootstrap.

These were printed as old-style implicit parameters before.

This is in preparation for merging it with `Implied`. We already compensate for
it when printing modifiers by eliding it when it appear with `Param`.

Synthesized creator expressions for classes passed all parameters
as normal arguments. This works no longer with `given` clauses. We
have use `given` arguments for those.

We now forbid given clauses followed by normal parameter clauses.
besides the syntactic awkwardness, there's also  the problem
of eta expansion. Example:
```
  trait Universe { type T }
  def f given (u: Universe) (x: u.T)
```
How should we tea expand `f`? The usual algorithm would give:
```
  (x: u.T) => (f given the[Universe])(x)
```
but that's ill typed, since `u` is not defined on the outside.

Ensure that every symbol has only one of the three flags
Implciit, Impled, Given. This is a prerequisite for simplifying
the flags in the future.

... to account for changes to implicit flags and new Exported flag.

`Implicit` will be changed to mean "old-style implicits".

Merge only method types that have the same companion

Rename isContextual -> isGivenApply in Apply nodes and FunProtos

It was a remainder from transparent methods, serves no purpose now.