Create multiple traits for Robj and wrapper methods.

[andy-thomason]

This is a major refactor of the implementation of the wrappers and Robj.

Currently, many of the wrappers implement Deref<Target=Robj> which makes making alternative
behaviour quite challenging.

For example, we would like to be able to index Integers with [i] and environments with ["str"]
ExternalPtr should behave like a Box enabling the contained element to be referenced
without calling a function.

This also starts to build a consistent interface for the wrappers which is expressed in traits.

We now only have two impl Robj blocks and one will be phased out.

Conversions between wrappers and Robj should be unified.

This PR should have very little effect on the visible interface, but should help us to start
cleaning up a bit.

There are a number of new traits that encapsulate existing functionality.

GetSexp

controls access to the underlying SEXP pointer.

Types

Generates the new Rtype and Rany enums

Length

Implements len()

Conversions

Likely to be phased out has a large number of as_xxx() conversions.

Rinternals

Has a number of internal R functions such as find_var()

Slices

Gives unsafe access to DATAPTR in vectors.

Operators

Implements R operators such as dollar()

[yutannihilation]

I guess this isn't the change intentionally included here, right?

[andy-thomason]

Yes. I imagine it got sucked into the vortex...

[andy-thomason]

I need to finish the Serialize work this month for the R foundation milestone but I'm being sucked into a batch of teaching
and project proposals.

[yutannihilation]

Oh, that's important.

[Ilia-Kosenkov]

Could we create a markdown in the docs that lists descriptions of all the traits? Like what it represents and where it is used/should be used in the future. It is rather hard to keep track of all the things, and this PR introduces even more traits, so perhaps it is a good place for start?

[andy-thomason]

Yes, we should make some better documentation now it is better organised.

We should write a guide to all the traits.

I would like to remove a lot of the old redundant impl Robj methods in favour
of a more consistent interface around the wrapper types.

[andy-thomason]

There are also a lot of our vector methods such as set_names() and elt() which should be in traits.

It should shrink down the function documentation somewhat, too.

[Ilia-Kosenkov]

Great. We have a mix of new and well-designed APIs and old outdated ones that honestly significantly slow my development process, so some docs or deprecation could be nice.

I will try to review this PR as soon as I can.