Add support for uploading files from memory + bug fixes

[MacDue]

This PR allows for uploading files directly from memory (a feature I needed).

uploadFile(Snowflake<Channel> channelID, uint8_t* buffer, size_t buffer_len, std::string message, std::string filename)

Note!
This PR changes the cpr repo to https://github.com/MacDue/cpr !
This is due to a bug with cpr where uploading files larger than 16,000bytes would corrupt the file -- which I repaired in my fork. Issue described here: libcpr/cpr#342

[MacDue]

• fixes for: C++20 support, missing includes, crashes in the error handler

[yourWaifu]

Can you separate the bug fixes into different pull request?

[yourWaifu]

Hello, are you there, I would be happy to accept bug fixes. However I want to consider the new feature separately.

[MacDue]

Not got around to it yet. It'll try to sort it out soon.

[yourWaifu]

I've added a number of you bug fixes, please fix the merge conflicts

[MacDue]

@yourWaifu the only merge conflict is the version of CPR I use (since I had to use a custom one to fix an upload bug), they recently merged a fix for that, but it looks like the version you use is still before that. I'll try switching the new official version soon & make sure that works (this branch is kind of changes for my bot -- not a super neat PR).

It currently has these changes:

• Uploading files from memory a SleepyDiscord::Buffer (with a shared_ptr to the data that allows it to work safely with SleepyDiscord::Async)
• Making Server::unavailable have three states (NOT_PRESENT, FALSE, TRUE) as detecting if it's not present can be used to tell if you're joining a new server or just reconnecting to a server.
• Previously merged Async fixes
• Updating asio to master (which fixed random high CPU bugs)

[yourWaifu]

I can't merge when there are merge conflicts, maybe try doing a git rebase.

[MacDue]

@yourWaifu I fixed it for you. I'm leaving the CPR version as you had it as I've yet to try the new version & don't want to link to my fork here. Though I suggest you look into updating it soon.

[yourWaifu]

I have updated cpr, I took a look at supporting buffer uploads but I found a problem. You Discord expects a filename if you want to attach it to an embed. CPR doesn't look like it has a way to set the filename of a buffer that I know of.

[yourWaifu]

oops nevermind my last statement about buffers and filenames, I didn't look at the libcurl docs hard enough

[MacDue]

You don't have to do it in libcurl, cpr::Buffer is a thing & it just takes the buffer data + a name.

[MacDue]

(the constructor of cpr::Buffer changed a bit since this PR, the name now has to be a std::string like: cpr::Buffer(data, data + m.buffer.length, std::string(m.name));)