string: report assert failures from assert caller level.

* ext/std/string.lua (assert): Call error with level 2, to report
errors from the assert caller not assert itself.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
tree: add specl specs, and fix simple API inconsistencies.

* specs/tree_spec.yaml: New file. Examples of how tree APIs
should behave.
* specs/specs.mk (specl_SPECS): Add specs/tree_spec.yaml.
* ext/std/base.lua (ileaves, leaves): Report non-table arguments.
* ext/std/tree.lua (clone, inodes, nodes, merge): Likewise.

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