let mut does not work with ~q matching

[eric-wieser]

This complains on each x :=

import Qq
open Qq Lean

example : MetaM Nat := do
  let mut x := 1
  if let ~q(Prop) := q(Type) then
    x := 2
  else
    x := 3
  return x

with error

x cannot be mutated, only variables declared using let mut can be mutated. If you did not intent to mutate but define x, consider using let x instead

Normally this would be legal:

example : MetaM Nat := do
  let mut x := 1
  if let true := false then
    x := 2
  else
    x := 3
  return x

This is almost certainly related to #21