SI-7747 Support class based wrappers as alternative through switch -Yrepl-class-based

Refactoring to reduce the number of if-else

Fix test.
SI-7747 Support class based wrappers clean up

Simplified the code paths to just use one of two `Wrapper` types
for textual templating.

Simplified the class-based template to use the same `$iw` name
for the both the class and the wrapper value.  In addition,
the $read value is an object extending $read, instead of containing
an extra instance field, which keeps paths to values the same
for both templates.

Both styles trigger loading the value object by referencing the
value that immediately wraps the user code, although for the
class style, inner vals are eager and it would suffice to load
the enclosing `$read` object.

The proposed template included extra vals for values imported
from history, but this is not necessary since such an import
is always a stable path. (Or, counter-example to test is welcome.)

The test for t5148 is updated as a side effect. Probably internal
APIs don't make good test subjects.

Modify -Y option message.
Filter dependencies from partest and scalacheck.

I previously messed this up in a24e7fa, which caused
partest classpath again to include multiple version
of scala-library and friends.

We should really automate enforcing uniqueness of packages.
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
Make parameters to implicit value classes private

So that they aren't offered as an autocomplete suggestion:

    implicit class Shouty(string: String) extends AnyVal {
      def SHOUT_! = string.toUpperCase + "!"
    }
    "". // autocompletion offers `.string` here

The original incarnation of value classes didn't allow this
sort of encapsulation, so we either invented goofy names like
`__thingToAdd` or just picked `x` or `self`. But SI-7859 has
delivered us the freedom to keep the accessor private.

Should we keep any of these accessors around in a deprecated
form?

The implicit classes in Predef were added in 2.11.0-M2
(c26a8db067e4f), so they are okay.

I think we can make reason that these APIs were both accidental
and unlikely to be interpreted as public, so we can break them
immediately.

    scala> Left(1).x
    res0: scala.util.Either[Int,Int] = Left(1)

    scala> import concurrent.duration._
    import concurrent.duration._

    scala> 1.n
    res1: Int = 1