std: accept lamda strings as an alternative to functions.

* specs/functional_spec.yaml, specs/io_spec.yaml,
specs/list_spec.yaml, specs/package_spec.yaml,
specs/string_spec.yaml, specs/table_spec.yaml: Specify behaviours
of API calls that accept functions when given a lambda string
instead.
* lib/std/functional.lua (Lambda, lambda): Move from here...
* lib/std/base.lua (Lambda, lamba): ...to here.
(Lambda): Save arguments in table fields.
(argcheck): Accept a valid lambda string in lieu of a Lua
function.
* lib/std/table.lua (sort): When passed a lambda string, pass the
associated function to core table.sort.
* lib/std/functional.lua (bind, case, collect, curry, filter)
(fold, map, memoize): Accept lamda strings in lieu of Lua
functions.
* lib/std/io.lua (process_files): Likewise.
* lib/std/list.lua (filter, foldl, foldr, map, map_with)
(zip_with): Likewise.
* lib/std/string.lua (render): Likewise.
* NEWS: Update.

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