Update special types.

[ehuss]

I think there are some types missing from special-types-and-traits.md. In particular, I think Fn, FnMut, and FnOnce are special. What is the criteria for being special? Is there anything else missing from the list?

Things to consider adding:

• cmp:
  • Eq
  • PartialEq
  • Ord
  • PartialOrd
• ops:
  • Add
  • AddAssign
  • BitAnd
  • BitAndAssign
  • BitOr
  • BitOrAssign
  • BitXor
  • BitXorAssign
  • Div
  • DivAssign
  • Fn
  • FnMut
  • FnOnce
  • Index
  • IndexMut
  • Mul
  • MulAssign
  • Neg
  • Not
  • Rem
  • RemAssign
  • Shl
  • ShlAssign
  • Shr
  • ShrAssign
  • Sub
  • SubAssign
• Misc:
  • Unpin — Automatically implemented on most types, but not otherwise "used" by the language?
  • Future
  • ManuallyDrop
  • IntoIterator
  • IntoFuture
• Unstable but maybe exposed in some way?:
  • ~~Termination — Exposed indirectly via main.~~ Done in #1194
  • Unsize / CoerceUnsized — Automatically implemented, involved in coercions?
  • UnsafeCell — Maybe for const eval?
  • StructuralPartialEq / StructuralEq — Needed for match checking?
• Unstable (not exposed?):
  • Generator, GeneratorState
  • Try
  • DispatchFromDyn ()
  • VaListImpl (va_list lang item)
  • Receiver (currently hard-coded in reference)
  • start
  • eh_personality
  • eh_catch_typeinfo
• Panic stuff — From a language perspective, is there anything relevant here?
• Relevance unknown:
  • Freeze
  • exchange_malloc
  • box_free
  • drop_in_place
  • oom
  • alloc_layout
  • MaybeUninit
  • align_offset — const eval?

[Havvy]

And type or trait is special if it requires or is a dependency of compiler machinery. The types you think are missing are probably the operator traits and we should probably duplicate that list into the page.