tree: remove std.object dependency.

No need to pull in all of std.object when loading std.tree.
* lib/std/tree.lua: Use base.prototype directly instead of via
re-exported object.prototype, which allows complete removal of
std.object dependency.
While we're here, use imported functions from locals to speed up
access a tiny bit.

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