SI-8016 Ensure that q”..$xs” is equivalent to q”{..$xs}”
SI-8009 Ensure that Idents preserve isBackquoted property
SI-6842 Make splicing less sensitive to precise types of trees

Previously trees that represent parameters, case clauses and
type variables had strictly defined ValDef, TypeDef and CaseDef
types which caused problems in compositionality.

Now this checks are moved to runtime so it's possible to pass
a tree that is CaseDef but has Tree type.
SI-7979 Fix quasiquotes crash on mismatch between fields and constructor
Move Liftable into the Universe cake; add additional standard Liftables

Previously we believed that having Liftable outside of the Universe will
bring some advantages but it turned out this wasn’t worth it. Due to
infectious nature of path dependent types inside of the universe one
had to cast a lot. A nice example of what I’m talking about is a change
in trait ArbitraryTreesAndNames.

Additionally a number of standard Liftables is added for types that are
available through Predef and/or default scala._ import: Array, Vector,
List, Map, Set, Option, Either, TupleN.
Provide a way for unapply macro to obtain a list of subpattens

This commit introduces internal attachment that allows unapply macros to
be aware of their sub patterns and tweak their expansion depending
on that info.

At the moment this is not possible due to the way pattern macros are
expanded:

	case MacroPat(inner1, inner2) => ...

During type checking this will expand as

	MacroPat.unapply(<unapply-dummy>)

Meaning that macro can’t see inner1 and inner2 in it’s macroApplication.
To circumvent this we attach that info as an attachment to the dummy.
Refactor Holes and Reifiers slices of Quasiquotes cake

This commit performs a number of preliminary refactoring needed to
implement unliftable:

1. Replace previous inheritance-heavy implementation of Holes with
   similar but much simpler one. Holes are now split into two different
   categories: ApplyHole and UnapplyHole which correspondingly represent
   information about holes in construction and deconstruction quasiquotes.

2. Make Placeholders extract holes rather than their field values. This
   is required to be able to get additional mode-specific information
   from holes (e.g. only ApplyHoles have types).

3. Bring ApplyReifier & UnapplyReifier even closer to the future where
   there is just a single base Reifier with mode parameter.

Along the way a few bugs were fixed:

1. SI-7980 SI-7996 fail with nice error on bottom types splices

2. Use asSeenFrom instead of typeArguments in parseCardinality.
   This fixes a crash if T <:< Iterable[Tree] but does not itself
   have any type arguments.

3. Fix spurious error message on splicing of Lists through Liftable[List[T]]
Introduce support for Unliftable for Quasiquotes

Unliftable is a type class similar to existing Liftable that lets
users to extract custom data types out of trees with the help of
straightforward type ascription syntax:

  val q“foo.bar(${baz: Baz})” = ...

This will use Unliftable[Baz] to extract custom data type Baz out of
a tree nested inside of the another tree. A simpler example would be
extracting of constant values:

  val q”${x: Int} + ${y: Int}” = q”1 + 2”
SI-7789 make quasiquotes deconstruct UnApply trees
Test tuple lifting and unfitting
Test unliftable not found scenario
Test usage of SubpatternsAttachment from a macro
Test possible quasiquote runtime failures
Address minor pull request feedback points

1. Use (x1, x2): (T1, T2) instead of (x1: T1, x2: T2)

2. More detailed error message for improper function argument

3. Fix typo

4. Completely remove LiftableClass symbol from definitions