maint: rename array to vector.

In mathematics "array" suggests the possibility of multiple
dimensions, and while one can simulate that with a std.array
of std.arrays, the name "vector" is a better fit for what this
class supports.
* lib/std/array.lua, specs/array_spec.yaml: Move from here...
* lib/std/vector.lua, specs/vector_spec.yaml: ...to here.
Rename symbols accordingly.
* build-aux/config.ld.in (file): Adjust accordingly.
* local.mk (dist_luastd_DATA, dist_classes_DATA): Likewise.
* specs/specs.mk (specl_SPECS): Likewise.
* specs/string_spec.yaml (render): Adjust Array using example to
Vector.

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