v0.5.4

nks to the following for their contributions:

- [@acdenisSK]
- [@drklee3]
- [@Lakelezz]
- [@vityafx]
- [@zeyla]

- [model] Add `Message::member` structfield ([@zeyla]) [c:0e1e8fb]
- [docs] Document example binding names for EventHandler method signatures
  ([@acdenisSK]) [c:08a7110]
- [model] Implement `Mentionable` for `CurrentUser` ([@zeyla]) [c:4a24c90]
- [model] Implement `From<CurrentUser> for User` and
  `From<&CurrentUser> for User` ([@zeyla]) [c:af7f176]
- [framework] Add option for bots to work only in certain channels ([@vityafx])
  [c:457a17e]
- [framework] Differentiate in help whether a command is unavailable in DMs or
  guilds ([@Lakelezz]) [c:89a18aa]
- [framework] Improve `Args` docs ([@acdenisSK]) [c:2603063]
- [model] Add `Message::mentions_user_id`, `Message::mentions_user`
  ([@Lakelezz]) [c:1162e68]
- [docs] Update voice example 06 to make joining join the command invoker's
  voice channel ([@drklee3]) [c:a80aab2]

- [framework] Fix a framework example so it makes sense ([@acdenisSK])
  [c:63fe032]
- [model] Remove deadlocking in `Member::highest_role_info` ([@zeyla])
  [c:c659bbd]
- [framework] Dispatch to a threadpool only if required ([@Lakelezz])
  [c:23c5398]
- [framework] Fix strikethrough behaviour ([@Lakelezz]) [c:32c3bed]

- [general] Fix links to the new repo location ([@Lakelezz], [@zeyla])
  [c:152fe3d] [c:0324e01]
- [framework] Switch to `str::match_indices` for some Args ops ([@acdenisSK])
  [c:cc6b567]
- [framework] Remove `if length == 1` branch in Args functions ([@acdenisSK])
  [c:6346975]
- [framework] Optimize `Args::find`, `Args::find_n` ([@acdenisSK]) [c:5ba521b]
- [framework] Revamp `Args` from the ground up ([@acdenisSK]) [c:ff9edc0]

[@acdenisSK]: https://github.com/acdenisSK
[@drklee3]: https://github.com/drklee3
[@Lakelezz]: https://github.com/Lakelezz
[@vityafx]: https://github.com/vityafx
[@zeyla]: https://github.com/zeyla

[c:0324e01]: https://github.com/serenity-rs/serenity/commit/0324e011f1ea0eed0709c92fe86319c812a42206
[c:08a7110]: https://github.com/serenity-rs/serenity/commit/08a71106748e356d2618e48d8797e6da60d7eb54
[c:0e1e8fb]: https://github.com/serenity-rs/serenity/commit/0e1e8fbbe564c23530a709a7ec407b08f63944e2
[c:1162e68]: https://github.com/serenity-rs/serenity/commit/1162e686592f23f4dc5ad509051858e453c82d1f
[c:152fe3d]: https://github.com/serenity-rs/serenity/commit/152fe3ded89c71580a9ab9d3bb05587abee97e72
[c:23c5398]: https://github.com/serenity-rs/serenity/commit/23c5398d8c6b0a3e5ad28cb43fadd48002195d3c
[c:2603063]: https://github.com/serenity-rs/serenity/commit/26030630bee7750c047b155708a62a03a6a5edf3
[c:32c3bed]: https://github.com/serenity-rs/serenity/commit/32c3bed1afa65d14a93d4e3d4e9e8471cfd77ced
[c:457a17e]: https://github.com/serenity-rs/serenity/commit/457a17e059395aab3d1a23bd1cfe6e01ea0b5a61
[c:4a24c90]: https://github.com/serenity-rs/serenity/commit/4a24c9004a1aac31eb552e5cdef6e986a6e903b3
[c:5ba521b]: https://github.com/serenity-rs/serenity/commit/5ba521b4e8bb5ff10bd83303436cd454f27c93ab
[c:6346975]: https://github.com/serenity-rs/serenity/commit/63469753a960431e5edacf520589493121f0e62d
[c:89a18aa]: https://github.com/serenity-rs/serenity/commit/89a18aa919d8c08cf9fba9a98ebe32c9fd59d5d4
[c:63fe032]: https://github.com/serenity-rs/serenity/commit/63fe032eaa29bbee51b6efd3f19392cc41b992e0
[c:a80aab2]: https://github.com/serenity-rs/serenity/commit/a80aab264b3d81c2457eb6bef755a5b0d1a88e4b
[c:af7f176]: https://github.com/serenity-rs/serenity/commit/af7f176101aea9bcf43551fbcd3261469bbc0b43
[c:c659bbd]: https://github.com/serenity-rs/serenity/commit/c659bbd756391fc26e9e862937ef77113ee892ed
[c:cc6b567]: https://github.com/serenity-rs/serenity/commit/cc6b5677d8e5ba61a53f7aeae26b66207db9fd3d
[c:ff9edc0]: https://github.com/serenity-rs/serenity/commit/ff9edc063fd987b9ba14c8d211ed26f6b480f751