Make defaults work on generic types

[Patitotective]

Trying to solve #13.
Here's an example proving that it should work:

import std/[macros, genasts]
macrodoIt(impl: untyped): untyped=result= impl
result[^1] =genast():
typeBleh[T] =object
x: T
procinitBleh[T](): Bleh[T] =Bleh[T](x: 100)
BlehtypeMyType {.doIt.} =objectechoinitBleh[int]()

Though right now, the code below fails:

import constructor/defaults
typeThingy[T] {.defaults: {}.} =object
c: T =1echoinitThingy[int]()

With:

$nimc--expandMacro:defaults trial.nim
...................................................................................................../trial.nim(4, 15) Hint: expanded macro: Thingy[T] {..} =type
Inner_536871044[T] =objectprocinitThingy[T](): Inner_536871044[T] =Inner_536871044[T](c: 1)
Inner_536871044 [ExpandMacro]
..../src/constructor/defaults.nim(63, 22) Hint: 'initThingy'isdeclaredbutnotused [XDeclaredButNotUsed]
.../trial.nim(7, 6) Error: undeclared identifier: 'initThingy'candidates (edit distance, scope distance); see'--spellSuggest': (5, 1): 'Thingy'