Minor cleanup for optax.projections.

[carlosgmartin]

Minor cleanup for optax.projections. Includes the following changes:

1. Change || to \| in docstring math formulas. The latter renders the double bars used in the notation for norms correctly.
2. Inline unnecessary function calls.
3. Replace float constants with ints, where possible, to avoid unnecessary conversions to float in some cases when the input is integral.
4. Remove unnecessary scale is None conditional in projection_simplex.
5. Remove unnecessary use of jax.lax.cond, which can be expensive, in projection_l2_ball.
6. Shorten unnecessarily verbose code.

[vroulet]

Thanks, minor comments left, then we can merge.
Did you build the docs to quickly check them too?