tibblify 0.2.0

Major rewrite of the tibblify package with lots of benefits:

• tibblify() is now implemented in C and therefore way faster.

• Support of column major format.

• Support for vectors as scalar lists and objects.

• Specification functions have been renamed

  • lcols() to tspec_df()
  • new specs tspec_object() and tspec_row()
  • lcol_int() to tib_int() etc

• tspec_df() gains an argument .names_to to store the names of a recordlist
  in a column.

• Added untibblify() to turn a tibble into a nested list, i.e. to reverse the action of tibblify().

• Added spec_combine() to combine multiple specifications.

• Added argument unspecified to tibblify() to control how to handle unspecified
  fields.

• Many bugfixes.