Clean-up of code & tests improvements

[42atomys]

Hi everyone !

When I Add GuildScheduledEvents on discordgo (#1032), I have seen several errors in the code and its organization that could be harmonized and tidied up.

I will list these changes below and if accepted, I would like to convert this into a milestone grouping several PRs to make these changes:

• In restapi.go uri and url is used without specific logic and maybe can be cleaned to url, following the URL convention (Add Guild Scheduled Event Support #1032 (comment), thanks to @FedorLap2006)
• In structs.go, we can think only contain struct, include custom struct helper and Marshal/Unmarshal function, this can be good to move functions on separated file (ex: marshal.go)
• A file called types.go contains only error, maybe rename it error.go
• Add a full test suit of package (Yes I think I have a mental problem with wanting to do this)
• Write a full test suite with a discord server used only for run tests on it to perform real-life end-to-end utilisation of discordgo

Any others changes, any discussions about it are welcome !

[FedorLap2006]

Not sure about structs and types. Additionally, I was planning to do a pr with moving stuff around to make it look better.
But it would be probably included only in v0.24.1

[42atomys]

Great, its can also a side milestone or include on 0.25 its not a high priority in itself

[42atomys]

In addition of my initial proposition :

• Write a full test suite with a discord server used only for run tests on it to perform real-life end-to-end utilisation of discordgo