Try/cycle detection

[odersky]

Some initial steps to take over former RefChecks functionality. Includes some cleanups in transforms.
The biggest change has to do F-bounded polymorphism. We now do cycle detection early and break
all legal cycles with a LazyRef type. Without this precaution lots of code would blow up with a stack overflow - checkbounds being one example.

[odersky]

@review by @DarkDimius and whoever else wants to have a look at it.

[samuelgruetter]

This PR seems to fix the StackOverflowError we got in #91, but here's another example which does not yet work:

object infpaths3 {

  object a {
    trait T { t =>
      type M <: t.g.N
      type T <: a.T
      val f: t.T

      trait U { u =>
        type N <: t.f.M
        type U <: a.x.g.U
        val f: u.U
      }
      val g: t.U
    }
    val x: a.T = ???
  }

}

On branch try/cycle-detection (#161) (and also on branch change/treecopying (#164)):

$ sbt "run examples/infpaths3.scala"
[error] examples/infpaths3.scala:5: error: illegal cyclic reference: upper bound T.this.g.N of type M refers back to the type itself
[error]       type M <: t.g.N
[error]              ^

but after outputting this, dotty runs into an infinite loop.

With scalac, the cycles are detected, and there's no infinite loop:

<console>:16: error: cyclic aliasing or subtyping involving type M
               type N <: t.f.M
                    ^
<console>:17: error: cyclic aliasing or subtyping involving type U
               type U <: a.x.g.U
                    ^