move for loop desugaring into tree gen

1. Move mkFor and it's dependencies to tree gen to make it accessible
   from tree builder and usable for quasiquotes.

2. Change intermediate representation of for loop enumerators from
   plain classes with positions into trees that encode the same data.
   This trees will also be used by quasiquotes as intermediate values
   obtained after for loop deconstruction.
implement inverse transformation to mkFor

This effectively reconstructs a sequence of enumerators and body
from the tree produced by mkFor. This lets to define bi-directional
SyntacticFor and SyntacticForYield constructors/extractors to work
with for loops.

Correctness of the transformation is tested by a scalacheck test
that generates a sequence of random enumerators, sugars them into
maps/flatMaps/foreach/withFilter calls and reconstructs them back.

Conflicts:
	src/reflect/scala/reflect/internal/BuildUtils.scala
add support for for loops and for enumerators to quasiquotes

1. q"for (..$enums) $body", q"for (..$enums) yield $body"

2. fq"..." quote to construct/deconstruct enumerators