vecvec 1.0.0

The vecvec class has been reimplemented using S7, replacing the previous vctrs
implementation. Existing code using vecvec objects should continue to work.

Breaking changes

• Removed new_vecvec(), which is replaced by class_vecvec for S7 consistency.
• Extension packages building new vecvec data types will need to use S7 classes
  that have the parent class class_vecvec.
• The internal data structure for vecvec objects has been overhauled. In
  practice these internal indices should not be used by users or developers,
  as the structure can change in the future to accommodate faster variants for
  special cases (#7).

New features

• vecvec classed objects now work as matrices and arrays (#15).
• is_vecvec() tests whether an object is a vecvec.
• class_vecvec S7 class and constructor for vecvec objects.
• Added vctrs methods for backwards compatibility and vctrs interoperability.

Improvements

• Substantially faster performance and reduced object size
• Adjacent compatible vectors are now automatically flattened when constructing
  or modifying a vecvec, reducing fragmentation.