deduplicate tuple tree creation code

Previously tuple tree generation code has been implemented in
three place: tree builder, tree gen, build utils. Now it's just
defined once in tree gen.
make internal implementation of universe.build less restrictive

1. Use protected instead of private to avoid needless lock-in

2. Use BuildImpl field type to expose non-protected members to the
   compiler (user-facing side in the reflection api stays the same)
add hasAttachment utility method to the internal api
add some post-typecheck tests for quasiquotes

Typecheck trees with toolbox and check that they are still matched
by corresponding quasiquote. Fix tuples and function types matchers
to account for different shape of trees after typing.
simplify imports in quasiquotes scalacheck tests
add support for importable attachments

Previously attachments weren't imported by importTree. Now a new marker
trait has been added that lets attachments to import themselves to the
new universe together with all their innards. Additionally a simpler
subtrait is defined to mark attachments that can be imported as-is.
make fresh names compare similar in quasiquotes

Otherwise it's annoying that trees that look the same are not equal
due to some fresh name hidden underneath. Due to this one test needs
to be changed to use plain equalsStructure.
change intermidiate representation of for loop enumerators

Encode values into real trees rather than non-tree case classes.
This is needed for re-usability of desugaring code between quasiquotes
and parser.
move for loop desugaring into tree gen
add syntactic combinators that represent enumerators
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.
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
re-implement hasAttachment directly in raw attachments
test legacy .filter support in for loop resugaring
add comments that explain new for loop enumerator encoding
add comments that explain FreshName extractor from Quasiquotes cake
Merge pull request #3090 from densh/pull/undo-for

Add support for For loops to quasiquotes