refactor: use `t[#t + 1] = v` rather than `table.insert (t, v)`.

Save a function call by factoring away calls to table.insert.
* lib/std/container.lua, lib/std/functional.lua, lib/std/io.lua,
lib/std/list.lua, lib/std/optparse.lua, lib/std/set.lua,
lib/std/strbuf.lua, lib/std/string.lua, lib/std/table.lua,
lib/std/tree.lua: Substitute accordingly.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>