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 - moves glob to its own library libglob.
Changes based on PR #11787. Updates .gitignore to ignore doc/glob.
auto merge of #11867 : dmanescu/rust/8784-arena-glob, r=huonw

In line with the dissolution of libextra - #8784 - this moves arena and glob into
their own respective modules. Updates .gitignore with the entries
doc/{arena,glob} in accordance.