add snapshot test capability without relying on string

[bobzhang]

fn expect[T:Show](
     obj : T,  ~value? : T , 
  ~loc : SourceLoc = _,
  ~args_loc : ArgsLoc = _
) -> Unit!InspectError {
  ...
}

• [ ] modernize inspect protocl using JSON data?

  ```patch
     - ~content : String = "",
     + ~content? : String,
  ```
  

• [ ] add expect API (using show first)

• [ ] decide whether we need a new dedicated trait called Repr or reuse Show

[lijunchen]

Potential Issue

Assume we already have expect! working as expected:

struct P {
  x: Int,
  y: Int
} derive(Show)

test {
  expect!({ x: 1, y: 2 })
}

After running the moon test -u, it becomes:

struct P {
  x: Int,
  y: Int
} derive(Show)

test {
  expect!({ x: 1, y: 2 }, value={ x: 1, y: 2 })
}

Now, if the definition of P changes (maybe common in practice), for example by adding a new field z, the value={ x: 1, y: 2 } argument will cause syntax errors. Users have to manually update or remove the value argument.

Changing a single definition may lead to amount test cases failing with syntax errors. The user experience is terrible in this scenario.

struct P {
  x: Int,
  y: Int,
  z: Int
} derive(Show)

test {
  expect!({ x: 1, y: 2, z : 3 }, value={ x: 1, y: 2 })
}