Need to Implement R's internal generics for JuliaObject

According to R's documentation for internal generic

The following primitive and internal functions are generic, i.e., you can write methods for them:

[, [[, $, [<-, [[<-, $<-,

length, length<-, dimnames, dimnames<-, dim, dim<-, names, names<-, levels<-,

c, unlist, cbind, rbind,

as.character, as.complex, as.double, as.integer, as.logical, as.raw, as.vector, is.array, is.matrix, is.na, is.nan, is.numeric, rep, seq.int (which dispatches methods for "seq") and xtfrm

In addition, is.name is a synonym for is.symbol and dispatches methods for the latter. Similarly, as.numeric is a synonym for as.double and dispatches methods for the latter, i.e., S3 methods are for as.double, whereas S4 methods are to be written for as.numeric.

Note that all of the group generic functions are also internal/primitive and allow methods to be written for them.

.S3PrimitiveGenerics is a character vector listing the primitives which are internal generic and not group generic. Currently as.vector, cbind, rbind and unlist are the internal non-primitive functions which are internally generic.

Start by reviewing the R documentation for internal generics quoted in the issue and locating JuliaObject in the JuliaCall repository. Compare the listed primitives, internal functions, and group generics with the methods JuliaObject currently exposes; done means the intended generic functions are implemented and their behavior is covered by the project's existing checks.