[WIP] Split up lib.rs into files

[TimonPost]

Apparently I was able to crate this branch directly. What do you prefer? Do you want me to fork this library or is this oke?

Don't worry about the LoC change count. This has to do with moving files around. You can skip all the added files. And the removed code in lib.rs.

Currently. For all the files and test I get the error:

error[E0603]: trait `SerdeDiff` is private
 --> examples/simple.rs:4:10
  |
4 | #[derive(SerdeDiff, Serialize, Deserialize, PartialEq, Debug)]

This has probably something to do with the proc macro. I am looking into this.

Changes

Fixes: #13

• No changes in logic.
• Same export API
• Moved files as described in Split up lib.rs #13.

I moved the re-exports like this:

#[doc(hidden)]
pub use apply::ApplyContext;
#[doc(hidden)]
pub use difference::{DiffContext, DiffPathElementValue};

pub use apply::Apply;
pub use config::Config;
pub use difference::Diff;

The usings in public export are used in serde-diff-derive crate. Previously all types were exported. But hidden for documentation. This PR only exposes the nessicarly types and hides the others. By doing this we introduce API breaking changes. Though the impact will be minimal because they were hidden for the documentation and we get better encapsulation. We do need to do a major patch.