Implement i8 and detect types correctly in MySQL

[tom-stytch]

Hi friends!

Came across an issue when trying to import from a MySQL source on a schema that made use of TINYINT columns, and found a couple of other discrepancies with what I understood to be the type translation as specified in the docs.

I am not very experienced in Rust, and I am primarily basing the work here on prior art.

We'll probably want the following:

• Extend the test scenarios to the Postgres cases, and/or confirm that these changes don't break some changes somewhere
• Bump a version - I think these might be considered breaking changes somewhat
• Add support for u8 as well?
  • This will also make the testing schema modification I made less nonsensical, I don't know what negative freckle count means 🤷

Thank you!

[iamwacko]

Oh oops, there's some warnings failing CI that aren't your fault. I'll get those fixed, then you can rebase them back into this PR.

At a cursory glance your code looks good, but I'll look more in depth later.

edit: fixes merged, you can sync with master (there might be .lock conflicts, sorry)