Rework ElimByName

[odersky]

Rework ElimByName, so that all transformations are now done in one phase, which
implements all of the following transformations:

1. For types of method and class parameters:

   => T becomes () ?=> T

2. For references to cbn-parameters:

   x becomes x.apply()

3. For arguments to cbn parameters

   e becomes () ?=> e

An optimization is applied: If the argument e to a cbn parameter is already
of type () ?=> T and is a pure expression, we avoid (2) and (3), i.e. we
pass e directly instead of () ?=> e.apply().

Note that () ?=> T cannot be written in source since user-defined context functions
must have at least one parameter. We use the type here as a convenient marker
of something that will erase to Function0, and where we know that it came from
a by-name parameter.

Note also that the transformation applies only to types of parameters, not to other
occurrences of ExprTypes. In particular, embedded occurrences in function types
such as (=> T) => U are left as-is here (they are eliminated in erasure).
Trying to convert these as well would mean traversing all the types, and that
leads to cyclic reference errors in many cases. This can cause problems in that
we might have sometimes a () ?=> T where a => T is expected. To compensate,
there is a new clause in TypeComparer#subArg that declares () ?=> T to be a
subtype of => T for arguments of type applications at any point after this phase
and up to erasure.

[odersky]

Can someone help figure out why the analyzer plugin test fails? @liufengyun do you have an idea?

[olhotak]

In ./sbt-test/sbt-dotty/analyzer-plugin/plugin/Analyzer.scala line 34:
override val runsBefore = Set(ElimRepeated.name)
Probably should be updated to ProtectedAccessors.

[odersky]

@olhotak That's probably it. Thanks for figuring it out!

[liufengyun]

ProtectedAccessors needs to be imported at line 17.

[odersky]

This PR now also contains a rework of RefChecks:

• Split out forward reference checks and cross version (i.e. experimental/deprecated/since) checks
  into their own miniphases. These have nothing to do with the core tasks of RefChecks.
• Move RefChecks earlier in the pipeline, so that it is not affected by ElimByName's questionable
  type manipulations.

RefChecks is one of the last things that we ported from Scala 2 and left largely unchanged. Despite being formally a miniphase, it was really the typical Scala 2 megaphase that mixed many different things. We could also go further and break out other things. There are still some checks that have nothing to do with overriding or overloading: checkImplicitNotFoundAnnotation and checkUnaryMethods, for instance.