Split up serde_generate into several crates

[weiznich]

Summary

This commit splits up serde_generate into several crates:

• One core crate containing the foundational types/traits
• One crate for each supported language
• One facade create reexporting support for different languages + the core crate. Support for specific languages is behind different feature flags
• One binary crate that just implements the command line application on top of the facade crate

The main motivation for this change is to reduce the number of unneeded dependencies for consumers that are:

• Interested in support of a only a partial set of the supported languages
• Using serde-generate as dependency instead of as binary crate (removes clap/yaml/…)

Fix #26

Test Plan

This all should work out of the box with the current testsetup as it "just" moves code around.

[jerel]

I like this! Definitely moves in a direction that is helpful for our project.

[weiznich]

I will try to land and make a release soon-ish. If you could fix the two comments, I would appreciate (otherwise I'll do it)

I can do the changes on Monday.

[ma2bd]

Almost there. Can you apply this commit to your branch? https://github.com/zefchain/serde-reflection/commit/d4871809148a6dfc7e0412d4c91e8d90243db97f

[ma2bd]

Published to crates.io !

[weiznich]

Thanks :tada: