SI-6844 restrict splicing in parameter position

Previously were a bit too permissive on how splicing in function
parameter position worked. This made confusing things like
possible:

   val x = TermName(“x”)
   q”def foo($x)”

Now you can either splice trees in that position (ValDefs) or
you have to provide type if you splice a name.
SI-8076 improve support for implicit argument list

This adds support for construction and deconstruction
of implicit argument list which was originally suggested
by @cvogt.

1. Splicing vale into implicit argument list automatically
   adds implicit flag to them:

     val x = q"val x: Int"
     q"def foo(implicit $x)"
     // <=> q"def foo(implicit x: Int)"

2. One might extract implicit argument list separately from
   other argument lists:

     val q”def foo(...$argss)(implicit ..$impl)" =
       q"def foo(implicit x: Int)
     // argss is Nil, impl contains valdef for x

   But this doesn't require you to always extract it separatly:

     val q”def foo(...$argss)" =
       q"def foo(implicit x: Int)
     // argss contains valdef for x
Tag synthetic unit with attachment

This makes it easy to differentiate unit inserted by a compiler vs unit
written by the user. Useful for quasiquotes and pretty printing.

Additionally SyntacticBlock extractor is changed to treat EmptyTree as
zero-element block.
Refactor reification of high-cardinality holes

New approach makes iterated function much more clear through aggressive
code reuse, recursion and large descriptive comment on top of it.
SI-7275 allow flattening of blocks with ..$

This commit extends current splicing rules to allow flattening of
trees into other trees.

Without such support it is impossible to correctly create vals with
patterns and use it in other location as they could expand into
multiple-statement blocks:

    scala> q"val (a, b) = (1, 2)"
    res0: reflect.runtime.universe.Tree =
    {
      <synthetic> <artifact> private[this] val x$1 = scala.Tuple2(1, 2):
    @scala.unchecked match {
        case scala.Tuple2((a @ _), (b @ _)) => scala.Tuple2(a, b)
      };
      val a = x$1._1;
      val b = x$1._2;
      ()
    }

    scala> q"..$res0; println(a + b)"
    res1: reflect.runtime.universe.Tree =
    {
      <synthetic> <artifact> private[this] val x$1 = scala.Tuple2(1, 2):
    @scala.unchecked match {
        case scala.Tuple2((a @ _), (b @ _)) => scala.Tuple2(a, b)
      };
      val a = x$1._1;
      val b = x$1._2;
      println(a.$plus(b))
    }
Address pull request feedback
Reify all blocks as syntactic blocks

Previous version of the pattern wasn't precise enough due to the
fact that synthetic units are now erased by syntactic block and
this could cause incorrect reification for cases like:

   val x = { val y = 1 }

Here syntactic block would extract one element but we still need to
reify it through syntactic block endpoint. So we can't filter based
on the number of elements extracted but rather filter on type of a
tree.
Addresses feedback from Jason

1. Adds tests for new synthetic unit stripping.
2. Marks implementation-specific parts of Holes as private.
3. Trims description of iterated method a bit.
4. Provides a bit more clear wrapper for q interpolator.
5. Refactors SyntacticBlock, adds documentation.
6. Makes q"{ ..$Nil }" return q"" to be consist with extractor.
SI-8182 Avert crash due to type args in pattern

Error out type args on binary op after emitting error.
Let the parse limp into the whirring blades.
Merge pull request #3418 from som-snytt/issue/8182-b

SI-8182 Avert crash due to type args in pattern
Merge pull request #3402 from densh/si/7275

SI-7275 allow flattening of blocks with ..$
Merge pull request #3374 from densh/si/6844-8076

SI-6844  SI-8076 improve handling of function parameters in quasiquotes