Align phase name of FirstTransform with class name
Drop modifiers as separate data from MemberDef trees

Typed MemberDef trees now take the modifiers from their symbol's data.
Make withMods produce Untyped trees

withMods should never be used to produce a typed tree, because typed
member defs get their modifiers from their symbols.
Factored out Positioned into separate file.
Merge pull request #220 from dotty-staging/refactor/DefTrees

Refactor/def trees