Support optim() and optimize() in nimbleFunction()

[fritzo]

See design doc.

[fritzo]

Experimental support was added in #318 . Work is ongoing.

[cwliu007]

Would you also add nlminb() which is more stable than optim() in my experiences? Thanks!

[perrydv]

Thanks for the suggestion. R's headers don't directly provide access to the C code for nlminb. Source code is here and we could make use of it.

We could also explore use of optimX in the future.

[fritzo]

This was implemented #348 and #367, was documented in #467, and is planned for release in 0.6-6.