refactor: use a string specifier instead of a table for export.

* specs/base_spec.yaml (export): Specify behaviours of export
function, particularly with argument errors.
* lib/std/base.lua (export): In place of an export name and a
table of expected argument types, parse a single `decl` argument
into an export name and table of argument types.
Support specified argument error behaviours.
* lib/std/math.lua (floor, monkey_patch, round): Simplify
accordingly.
* lib/std/functional.lua (bind, case, collect, compose, curry)
(eval, filter, fold, map, memoize): Likewise.

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