tibblify 0.3.0

• In column major format all fields are required.

• Fixed a memory leak.

• tib_vector() is now uses less memory and is faster.

• tspec_*(), tib_df(), and tib_row() now discard NULL in .... This
  makes it easier to add a field conditionally with, for example
  tspec_df(if (x) tib_int("a")).

• tib_variant() and tib_vector() give you more control for transforming:

  • transform is now applied to the whole vector.

  • There is a new elt_transform argument that is applied to every element.

• New tspec_recursive() and tib_recursive() to parse tree like structure,
  e.g. a directory structure with its children.