Add Outline API, and make structs derive more traits within reason

[yoanlcq]

Greetings,

The title says it all :) That part of the FreeType2 API is documented here.
I made sure to double-check the function signatures.

While I was at it, I allowed myself to make most structs a bit more interesting by making them derive more traits, while being careful not to derive Copy and Clone for structs that are purposefully marked as "uncopyable".
This part of the PR may be controversial, but I think that all structs should at least derive Debug, because otherwise it's a pain when making higher-level APIs.

Thanks in advance!

[bvssvni]

Looks good.

Appveyor is failing for some reason.

[yoanlcq]

Trivial Google-fu suggests that there's a mismatch between two Mingw-w64 versions in the AppVeyor script. (issue here)

I suspect it's one (or both?) of these :

• PATH is set to C:\msys64\mingw64\bin;C:\Rust\bin;
• pacman mingw-w64-x86_64-freetype;

In either case, this could be because the mingw64 bundled with rust-CHANNEL-x86_64-pc-windows-gnu doesn't match some other that involved in the operation.
I'll try fixing for now.

[yoanlcq]

There we go! I hope you don't mind that the AppVeyor build now uses the msvc target. I felt that the mingw64 issue would have been a nightmare to deal with, because we don't have much control over the mingw64 version used by msys64 AND the possibly different one used by the freetype DLL from pacman.

Either way, I'm not willing to spend any more time digging for a "real" solution for the gnu target.
A real fix would involve having precise control over the mingw64 version used in every part of the process, which I predict would be a time sink with negligible return on investment.

[bvssvni]

Nice! Merging.