Panic Safety and Mild Cleanup

[cwfitzgerald]

Resolves #11.

This PR removes all instances of panics where the user didn't ask for a panic. I've tested the tests and against my codebase using it, and everything seems still in order.

Notable changes

• Added ObjError, MtlError, and MtlMissingType
• Refactored Obj::{parse_vertex, parse_texture, parse_normal} to return their output instead of modifying state. Combined them into parse_two and parse_three to reduce duplicate code.
• Added hack to allow filenames with spaces as the mtllib/texture names. (Explanation in comment)
• Added private function Obj::load_single_mtl to make error handling more concise and remove duplicate code.

Breaking Changes:

• Everything now returns a ObjError or MtlError instead of a io::Error
• Neither ObjError nor MtlError implement Display. Should they?

Now that I've dug a bit into the codebase, there are a couple of improvements I still want to make. Feel free to comment on any of them 😄. Vaguely in order of importance:

• Run rustfmt on the codebase & verify rustfmt in CI
• Create function Obj::load_mtl_fn that takes a function which takes the base directory and the mtllib name and returns something that impl Read. This will allow special file lookup or file loading if need be. Obj::load_mtl will just call Obj::load_mtl_fn with a function that uses how it currently resolves now. Will file an issue about this tmmr.
• Document all the things.
• Maybe implement #2.

[cwfitzgerald]

All changes should be addressed :)

[kvark]

Are the commits buildable independently? Otherwise we'll squash it (no bors here)

[cwfitzgerald]

I think they all build. I don't recall running tests on all of them but I only committed once I got things working.