Add lib.rs so difftastic can be used as a library (& serde)

[anchpop]

Sorry for mixing two changes together like this.

1. I'm trying to use difftastic for one of my own project, but I needed it as a library, so I added a lib.rs and moved some relevant functionality to it. main.rs works as before.
2. I also needed Serialize and Deserialize on DiffResult so I added that too.

[petr-tik]

looks related to the issue I opened a week ago
#216

[cherryblossom000]

Maybe add an optional feature that can be enabled for serde support so that most users don't have to build serde and its dependencies when compiling the binary

[anchpop]

@petr-tik hope it helps. I'm curious what you're using difftastic for. (Until this gets merged, you can always depend on my repo).
@cherryblossom000 that's a good idea. I'm not too sure how to use cargo features though, so it may be a small wait until I implement that

[luanraithz]

I also have a use case for using this as a library, is this PR dead?

[jzeimen]

Same here, I also need the library use case. Anything we can do to help get this done @Wilfred ? Happy to help, but want to know what we need to do to get this over the line besides re-basing.

The use-case is to be able to pass 2 strings representing LHS and RHS files, get the results back to know what lines have changed and what parts of the lines are novel w/ start and end position. Then act on certain scenarios programatically. This avoids writing to disk and parsing the existing CLI output.

There's also the use-case for json output in PR #158. These seem related. The library method and json should have the same output shape. That one is close but didn't tell me which tokens were novel, locally I added it and it works but this library scenario would be much better.

[brurucy]

Any updates on this?

[wongjiahau]

I'm also looking to integrate difftastic into my editor

[brurucy]

@Wilfred sorry to tag you here :) but it would be immensely useful to more than one person if this could be done.

[CHE1RON]

Since it's been a while now, maybe @anchpop and @Wilfred can come together and have a look at this? 🤔 It would be much appreciated if this makes it into the library!

[leontrolski]

I'd also like to wrap difftastic with pyo3 (similar Python libs in this space are pretty slow)

[wongjiahau]

It seems like the issue was mistakenly closed by bf4331c, perhaps due to a typo of #231 in the commit message?