Add todo!() macro

[matklad]

The primary use-case of todo!() macro is to be a much easier to type
alternative to unimplemented!() macro.

EDIT: hide unpopular proposal about re-purposing unimplemented

Details

However, instead of just replacing `unimplemented!()`, it gives it a more nuanced meaning: a thing which is intentionally left unimplemented and which should not be called at runtime. Usually, you'd like to prevent such cases statically, but sometimes you, for example, have to implement a trait only some methods of which are applicable. There are examples in the wild of code doing this thing, and in this case, the current message of `unimplemented`, "not *yet* implemented" is slightly misleading.

With the addition of TODO, you have three nuanced choices for a
!-returning macro (in addition to a good-old panic we all love):

• todo!()
• unreachable!()
• unimplemented!()

Here's a rough guideline what each one means:

• todo: use it during development, as a "hole" or placeholder. It
  might be a good idea to add a pre-commit hook which checks that
  todo is not accidentally committed.

• unreachable!(): use it when your code can statically guarantee
  that some situation can not happen. If you use a library and hit
  unreachable!() in the library's code, it's definitely a bug in the
  library. It's OK to have unreachable!() in the code base,
  although, if possible, it's better to replace it with
  compiler-verified exhaustive checks.

• unimplemented!(): use it when the type checker forces you to
  handle some situation, but there's a contract that a callee must not
  actually call the code. If you use a library and hit
  unimplemented!(), it's probably a bug in your code, though
  it could be a bug in the library (or library docs) as well. It is
  ok-ish to see an unimplemented!() in real code, but it usually
  signifies a clunky, eyebrow-rising API.