refactor: merge lib/std/base files into std.base.

* lib/std/base/functional.lua (callable, collect, reduce): Move
from here...
* lib/std/base/string.lua (copy, render, split): ...here...
* lib/std/base/tree.lua (leaves): ...and here...
* lib/std/base.lua (callable, collect, reduce, copy, render)
(split, leaves): ...to here.
Adjust all callers.
* lib/std/base/functional.lua, lib/std/base/string.lua,
lib/std/base/tree.lua: Remove files.
* local.mk (luastdbasedir, dist_luastdbase_DATA): Remove.

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