SI-8008 Make q”f(..$xs)” only match trees with Apply node

Previously it also matched other nodes but returned Nil as value of xs.
This behavior was added for sake of consistentcy with q”f[..$ts]”. On
the other hand q”f[..$Nil]” == q”f” but q”f(..$Nil)” == q”f()” not q”f”.
Due to this deconstruction/construction symmetry was broken.

On the other hand applications also have q"f(...$xss)" option which is
infact similar to q"f[..$ts]". Splicing Nil into it also results in q"f".
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 unlifting
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
Merge pull request #3255 from densh/pr/unliftable-3

Introduce Unliftable for Quasiquotes (take #3)
Merge pull request #3241 from retronym/ticket/8054

SI-8054 Fix regression in TypeRef rebind with val overriding object
Improve shadowed implicits detection, reporting.

Rather than using `typedIdent` to detect shadowing, do a direct
lookup in the scope with `Context#lookup`.

In the process, we simplify the manner in which the implicit tree
is created, but delegating the job to `TreeGen`, after extending
it to support creating `Ident`s with a name different to the
referenced symbol, as can happen with aliased imports.

Before this change, the reason for shadowed implicit was obscured
by the inability to typecheck the shadower.

    % scalac -Xlog-implicits neg/t7788.scala
    conf is not a valid implicit value for T[String] because:
    hasMatchingSymbol reported error: polymorphic expression cannot be instantiated to expected type;
SI-7788 Predef.conforms can circumvent implicit shadowing

Regular implicits play by the rule that they are only usable
implicitly if they could be used explicitly. This means that
they cannot be shadowed.

`Predef.conforms` is a bit special: it already has one special
rule for implicit search, in that it can't be used for implicit
views.

This commit adds a second special case: it cannot be shadowed
by another non-implicit member.
SI-7788 Refactoring in typedImplicit1

  - route checks for Predef.conforms through one method
  - handle all the failure cases of `NameLookup` at once
  - Use string interpolation
  - outsource generation of the Ident or Select tree to,
    TreeGen.

More to do in here, but I'm trying at least to leave things
no messier than when I arrived.