specs: decouple spec_helper.lua from std.table and std.functional.

* specs/spec_helper.lua.in (bind): Use our own implmentation of
bind, otherwise if std.functional becomes unloadable, the whole
specl suite is unusable.
(totable): Likewise for std.table.totable.
(set): No need to rely on std.set, when a simple table index
dereference works equally well.

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