refactor: move language features to a new  `std.lua` module.

* specs/functional_spec.yaml (case, eval, lambda, memoize): Move
from here...
* specs/lua_spec.yaml: New file. ...to here.
* specs/std_spec.yaml: Adjust accordingly.
* specs/specs.mk (specl_SPECS): Add specs/lua_spec.yaml.
* build-aux/config.ld.in (file): Add lib/std/lua.lua.
* lib/std/base.lua (lambda): Move from here...
* lib/std/lua.lua (lambda): New file. ...to here.
* local.mk (dist_luastd_DATA): Add lib/std/lua.lua.
* lib/std/functional.lua (case, eval, lambda, memoize): Move from
here...
* lib/std/lua.lua (case, eval, lambda, memoize): ...to here.
* lib/std/string.lua (pickle): Adjust LDocs eval cross reference.
* NEWS: Update.

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