feat: use serde for de/serializing GuildId

[seqre]

Serde is smart and can handle a lot of de/serializing magic. This PR aims to reduce extraneous manual implementations in favor of Serde's existing functionality.

I know deserializing works in this case (Serde is fully capable of taking a String as input and going through multiple transformations of String -> u64 -> GuildId); however, I need to do more testing for serializing (some additional Serde magic might be needed to force it to serialize to String, nothing though that Serde shouldn't be capable of); therefore, this PR is in draft mode for now.

[seqre]

Funny thing, disregarding the fact Serde deserializes to numbers (shown below), the tests pass, and config loads lol

[guilds.420]
tournaments_watcher_channel_id = "69"

[guilds.69]
tournaments_watcher_channel_id = "2137"

[codecov]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (c119696) 43.56% compared to head (5bff7fd) 45.38%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #206      +/-   ##
==========================================
+ Coverage   43.56%   45.38%   +1.82%     
==========================================
  Files          22       22              
  Lines        1398     1430      +32     
==========================================
+ Hits          609      649      +40     
+ Misses        789      781       -8     

Flag	Coverage Δ
rust	45.38% <95.65%> (+1.82%)	⬆️

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Files	Coverage Δ
chombot/src/config.rs	89.74% <100.00%> (+13.27%)	⬆️
chombot/src/tournament_watcher.rs	0.00% <0.00%> (ø)

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[seqre]

Actually, digits are valid keys for TOML tables/dictionaries:

• Guide: https://github.com/toml-lang/toml/blob/main/toml.md#keys
• Table definition: https://github.com/toml-lang/toml/blob/main/toml.abnf#L231
• Key definition: https://github.com/toml-lang/toml/blob/main/toml.abnf#L48-L56

toml-rs seems to support only String as the Table key, but somehow it works.

[seqre]

It just works ™️

[m4tx]

Thanks for the nice simplification!