Merging nnkArgList and nnkBracket

[mratsim]

This is a follow-up on the discussion here: #8715 (comment).

For me this is the main reason I use nnkArgList, example taken from #7737.

import macros, typetraits, sequtils

proc getSubType*(T: NimNode): NimNode =
  echo getTypeInst(T).treerepr # <---- Output comes from here
  result = getTypeInst(T)[1]

macro typed_helper(x: varargs[typed]): untyped =
  let foo = getSubType(x[0])
  result = quote do: discard

macro untyped_heavylifting(x: varargs[untyped]): untyped =

  # var containers = nnkArgList.newTree()
  var containers = nnkBracket.newTree()

  for arg in x:
    if eqIdent(arg[0], "in"):
      containers.add arg[2]

  result = quote do:
    typed_helper(`containers`)

var a, b, c: seq[int]

untyped_heavylifting z in c, x in a, y in b:
  discard

The getSubType proc returns:

# Case nnkArgList
BracketExpr
  Sym "seq"
  Sym "int"

and

# Case nnkBracket
BracketExpr
  Sym "array"
  Infix
    Ident ".."
    IntLit 0
    IntLit 2
  BracketExpr
    Sym "seq"
    Sym "int"

i.e. varargs[typed] needs nnkArgList for proper type resolution.
cc @Araq @zah @LemonBoy

Related to #7719/nim-lang/RFCs#44 (Working with types in macro is difficult)

[LemonBoy]

I'd argue that we just need a bit of consistency here, let's use nkArgList for argument lists and nkBracket for everything else instead of this awkward situation where you can mix&match nodes.