Add tests; make some minor code changes

[lukesonnet]

1. Changes default returnopt to FALSE and actually returns it when the user wants it. We could remove this later if we want, but I think returning the value and the optim metadata is useful for people trying different hyperparameters.

2. Makes the package overall a bit more CRAN friendly. Much more has to be done here, but just in terms of how we import, export, and register functions. This is because the previous solution throws warnings in R CMD check, which will slow down CRAN submission. Note, it looks like someone already did this in July for the main branch of KRLS to keep it on CRAN.

3. Adds tests. The important one demonstrates that indeed the analytical gradient and hessian we have in the .cpp are correct, using numerical derivatives to test. You can see this in tests/testthat/test_krlogit_fns.R

4. Extends our CI to include test coverage and more R releases and operating systems.