alien: implement a subset of alien to simplify wrappers.

* specs/alien_spec.yaml: Specify behaviour for a useful subset
of the alien APIs.
* specs/specs.mk (specl_SPECS): Add specs/alien_spec.yaml.
* local.mk (dist_modules_DATA): Add doc/modules/std.alien.html.
* lib/std/alien.lua: New file, provide pure Lua implementation
of alien.array, alien.memmove and alien.memset.
* build-aux/config.ld.in (files): Add lib/std/alien.lua.
* local.mk (dist_luastd_DATA): Add lib/std/alien.lua.

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