Use custom `derive` macros for all derive functionality

[kupiakos]

This would replace #11 and #12.

The current detection of Debug can be kept as an ergonomic bonus, since std/3rd party derives using the variants of an open enum are unlikely to work correctly.

Design

The #[open_enum] macro declares an #[open_enum_variants = { Name0 = 0, Name1 = VALUE1, ... }] inert attribute on the emitted struct. This attribute is consumed by the derive macros declared in open_enum, so most will work whether the derive macro is interpreted before or after #[open_enum].

As done today, PartialEq and Eq are always implemented on open enums so they work in match.

These common derive macros work without interception:

• PartialOrd
• Ord
• Hash

derive macros to consider, including additional nice-to-haves:

• [ ] ToStr - to an Option<&'static str> with the name, or None if not a known variant. When allow_alias is enabled, the first defined variant wins.
• [ ] ToCStr - to an Option<&'static ffi::CStr> with the name, or None if not a known variant. When allow_alias is enabled, the first defined variant wins. Might want to gate this on optimize_for_size.
• [ ] Debug is captured specially today, this would still be captured and replaced with the open-enum derive.
• [ ] FromRepr - implements From<$repr> for the open enum. Only works when the repr is explicit
• [ ] TryFromKnownRep - implements TryFrom<$repr> for the open enum, checking for variants. Only works when the repr is explicit
• [ ] ToRepr - implements Into<$repr> for the open enum. Only works when the repr is explicit
• [ ] FromStr
• [ ] FromPrimitive - implements FromPrimitive, allowing only known variants.
  • [ ] Requires a #[from_known_only = $bool] attribute
  • [ ] Requires the optional num_traits = "0.2" feature
• [ ] Default #23

Size optimization

• [ ] A #[cfg(feature = "optimize_for_size")] can switch between fast lookup and small code size.
• [ ] Names can be stored with an array of pointer-to-c-strings instead of array of pointer + size. However, this does make ToStr more slow than it otherwise would be.
• [ ] Names and values can be sorted so that a binary search can be performed for either lookup. Both the algorithm and the extra sorted array would be extra code size.
• [ ] "Is known value" and "Lookup name index" could be optimized for simpler non-contiguous ranges by using e.g. a u64 bit array.

Performing the above operations at const time if possible is strongly preferred since it can perform these optimization with nonliteral discriminants.

[kupiakos]

Alternative design: #[open_enum] implements an EnumInfo trait that contains the necessary info to implement everything.