Project restructure! (issue #3)

[vivyir]

Restructured the project according to the resources you provided, unfortunately I got carried away and put everything into one massive commit, but below is a relatively accurate summary of everything that was changed (copied verbatim from the commit message with slight edits), please have a look over the code aswell, I'd love any feedback you may have, thanks!

Notes:

• semver wasn't increased cuz im not sure if it should be yet (because the api interface and whatnot is still the same)
• api compatability with old code was tested, tests are also all successful
• formatted with the provided rustfmt config
• as outlined in the edits, some fields were made public because I couldn't figure out how to make them work without doing so, they aren't visible in the docs but i'm still not sure.

Edits:

• it will now compile on stable rust
• in order to make it compile on stable rust a new function that mimicked the assume_init function had to be made: "assume_init_box" (in common.rs)
• all the code was separated into 3 modules, common, encoder and decoder (visibilities and api were kept as is, old source code should still be compatible with the restructure)
• function "new" added for OggPacket to make stuff cleaner
• VorbisInfo's (in common/vorbis_data.rs:20-21) field was made public
• VorbisComments's (in common/vorbis_data.rs:196-197) field was made public
• instead of collect_into, extend was used (decoder/decoder_data.rs:39-41)
• VorbisEncodingState's (encoder/encoder_data.rs:13-18) fields were made public
• all occurences of .assume_init() being used on Box objects were replaced with calls to the new "assume_init_box" function

[AlexTMjugador]

Splendid work! I genuinely appreciate the effort you put into making such a well-written PR! :heart:

I'm a bit busy at work now, but I'll arrange the proper time and attention to review your PR as soon as possible.

Edit: I authorized the CI workflows to run, and it looks like Clippy outputted some warnings. You could take care of them in the meantime to save on reviews!