User data feature

[B-Reif]

This PR adds some support for parsing user data chunks and attaching them to other data.

Changes include:

• Adds optional user data field to Cel, Layer, Tag, and AsepriteFile structs.
• Adds a new UserDataContext enum to ParseInfo, which stores some information about the most-recently parsed chunk. When parsing a UserData chunk, the context is used to attach the user data to other existing content.
• Adds a new InternalError case to the AsepriteParseError enum. This would only be returned in case of some violated invariant. For now, it's only returned if the UserDataContext holds a bad id.
• Refactors ParseInfo to store layers during parsing, to support attaching user data.
• Refactors some methods to use CelId more.

A few questions:

• The Aseprite file spec mentions that a user data chunk can follow any other chunk. The Aseprite UI only exposes user data fields in some of these contexts. What kinds of chunks should we support user data on?
• Currently the 'color' field in UserData is an array of bytes. Since we're already exposing image::Rgba, should we change this field to that type?

[alpine-alpaca]

Regarding your questions:

• We only need to support the user data chunks that are actually exposed by the UI. I can imagine users storing some meta data for custom tools in these chunks, but I expect files to only be authored by hand. So, no need to support everything that the file format theoretically allows. If someone has a specific use case that's currently not supported they'll create an issue and we go from there.

• The color in user data should be an Rgba since we're already exposing that through other parts of the API.

[B-Reif]

Ok, made some changes:

• removed commented-out old code
• use image::Rgba in UserData
• Removed the LayerParseInfo struct. The Vec<LayerData> is now converted to LayersData during validation.
• Added tests for UserData colors

[alpine-alpaca]

Can I close this since you created the merged PR?