extra: move arena to libarena

In line with the dissolution of libextra - #8784 - moves arena to its own library libarena.
Changes based on PR #11787. Updates .gitignore to ignore doc/arena.
extra: move glob to libglob

In line with the dissolution of libextra - #8784 - moves glob to its own library libglob.
Changes based on PR #11787. Updates .gitignore to ignore doc/glob.