Implemented stronger type inference for enums and literals

[beef331]

This PR aims to strenghten the type inference when the type can be resolved from the left hand.
Following is the added features:

type
  A {.pure.} = enum
    left, right, up, down
  B {.pure.} = enum
    left, right, up, down
  Test = object
    case kind: A
    of left:
      a: int
    of {right, down}:
      b: float
    else:
      c: string
  ASet = set[A]

let test = A.left
case test:
of left: discard
of up: discard
else: discard

var
  a: array[4, A] = [left, right, down, up]
  b: array[4, B] = [left, right, down, up]
  c: ASet = {left..right, down, up}
  d: A = left
  e: array[4, byte] = [255, 2, 3, 4]

[Araq]

This needs an RFC and the implementation looks off-putting. (Which is not your fault, the code follows the existing style, but we have to do better than this, a little type flage here and there, a couple of more edge cases, what could go wrong...)

[beef331]

Closing with nim-lang/RFCs#373 being made.