Corrects behavior of finalResultType.

The implementation had come to depend on finalResultType
accidentally doing things beyond its charter - in particular,
widening types. After hunting down and fixing the call sites
depending on the bugs, I was able to rewrite the method to do
only what it's supposed to do.

I threw in a different way of writing it entirely to suggest how
some correctness might be obtained in the future. It's a lot
harder for a method written like this to break.
Widen type of outer accessor.

This is the key ingredient so TypeTree(sym) can resist
widening the type.
Reducing variation of tree creation methods.

TreeDSL has no future - it was always a temporary measure
waiting for something like quasiquotes to come along. In this
commit I cull as much of it as I can, especially the delicate
matter of creating new DefDefs and ValDefs, which I completely
turn over to the old style creators.

I unified all the symbol-based DefDef and ValDef creators under
a single method, since it was yet another place where ctrl-C and
ctrl-V were being punched with glee. Was beaten to the punch on
adding copyTypeDef to fill out the *Def creators.

Eliminated as many redundant positioning calls as I could find.
If you are creating a DefTree tree based on a symbol, it will
always have an atPos(sym.pos) { ... } wrapped around it. You
don't need another one.

All of this is motivated by positions work: positions are
assigned in so many places and in such an ad hoc fashion that
it is impossible to bring consistency to that without first
bringing some consistency to tree creation.
Merge pull request #2931 from paulp/pr/TreeDSL

Reducing variation of tree creation methods.