Add zoom line search.

[mblondel]

This PR is the first of a series of PRs to integrate zoom line search in JAXopt. It uses the code of @Joshuaalbert currently in JAX core. It supports only 1d arrays for parameters at the moment.

To be done in other PRs:

1. write more correctness tests, e.g. tests for isolated functions such as quadmin or cubicmin
2. simplify the code when possible
3. add pytree support, either natively or via flattening
4. benchmarks: backtracking vs. zoom, SciPy vs. JAXopt on various problems (both convex and nonconvex)
5. use our iterative solver API instead of using a lax.while_loop (this will give access to individual steps of the line search)

CC: @shoyer @jakevdp @junpenglao

[mblondel]

This is now ready for review. I'll tackle the aforementioned improvements separately, as I think it's important to have a copy of the original code in the commit history. A benchmark on a small synthetic dataset (10,000 samples, 200 features) looks good:

[fabianp]

looks good to me, thanks!