Add `empty` method for `r_vector` classes.

[jimhester]

The C++ core guidelines recommend using empty() methods where available, because this makes the containers more generic, e.g. you don't have to know that an empty container means the size is 0.

https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#t143-dont-write-unintentionally-non-generic-code

We should add empty() methods that return size() > 0.

We should probably use empty() rather than is_empty() to match the std::vector APIs, when in doubt I have been trying to stick to that.

[jimhester]

@sbearrows this might be another good one to work on.

You will have to add a declaration in the class declaration

cpp11/inst/include/cpp11/r_vector.hpp

Line 109 in 4ef13b7

R_xlen_t size() const;

and then add a definition further down the file

cpp11/inst/include/cpp11/r_vector.hpp

Line 382 in 4ef13b7

template <typename T>