maint: revert automatic lambda string compilation.

* lib/std/base.lua (lambda): Core Lua APIs require actual
functions, so to be useful for passing lambda strings to core
APIs and stdlib APIs alike, return a raw function rather than a
functable.
(argcheck): Don't accept a compilable lambda string where a
function argument is expected.
* lib/std/debug.lua (lambda, argcheck): Adjust LDocs.
* lib/std/functional.lua: Likewise.
(bind, case, collect, compose, curry, filter, fold, map)
(memoize): Remove lambda argument compilation.
* lib/std/io.lua (process_files): Likewise.
* lib/std/list.lua (filter, foldl, foldr, map, map_with)
(zip_with): Likewise.
* lib/std/package.lua (mappath): Likewise.
* lib/std/string.lua (render): Likewise.
* lib/std/table.lua (sort): Likewise.
* specs/debug_spec.yaml, specs/functional_spec.yaml,
specs/io_spec.yaml, specs/list_spec.yaml,
specs/package_spec.yaml, specs/string_spec.yaml,
specs/table_spec.yaml: Adjust lambda specs.
* NEWS: Update.

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