SI-1803, plus documentation and cleanups in Namers, mainly in typeSig

- when typing (and naming) a ValDef, tpt and rhs are now type checked
in the same context (the inner / ValDef context). this does not change
any behavior, but is more uniform (same as for DefDef). martin told me
(offline) that this change is desirable if it doesn't break anything.
(it doesn't).

- typeSig is now more uniform with a separate method for each case
(methodSig, valDefSig, etc). methodSig was cleaned up (no more variables)
and documented. the type returned by methodSig no longer contains /
refers to type skolems, but to the actual type parameters (so we don't
need to replace the skolems lateron).

- documentation on constructor contexts, type skolems

- more tests for SI-5543

Author: lrytz

test/files/pos/t1014.scala

@@ -1,6 +1,8 @@
 import scala.xml.{NodeSeq, Elem}
 
-class EO extends App with Moo{
+class EO extends App with Moo {
+  // return type is Flog, inherited from overridden method.
+  // implicit conversions are applied because expected type `pt` is `Flog` when `computeType(rhs, pt)`.
   def cat = <cat>dog</cat>
 
   implicit def nodeSeqToFlog(in: Elem): Flog = new Flog(in)