Make {.mockable.} useable with generics

[PhilippMDoerner]

This currently breaks:

import std/[genasts, macros]

macro mockable*(p: typed): untyped =
  when defined(mock):
    let
      pName = ident($p[0])
      newName = ident($p[0] & "Base")
      oldPrc = p.copyNimTree

    oldPrc[0] = newName
    result = genast(pName, oldPrc, newName):
      oldPrc
      var pName* = newName
  else:
    result = p


proc toString*[T](x: T): string {.mockable.} = $x

/home/philipp/dev/playground/src/playground.nim(18, 35) template/generic instantiation of `mockable` from here
/usr/lib/nim/std/genasts.nim(87, 13) Error: 'toStringBase' doesn't have a concrete type, due to unspecified generic parameters.