Array: add a set method.

* lib/zile/specs/Array_spec.yaml (set): Specify a new method to
set a contiguous block of elements to a given value.
* lib/zile/Array.lua (set): Implementation to meet specs.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
MutableString: rename set_len method to realloc.

* lib/zile/MutableString.lua (set_len): Rename from this...
(realloc): ...to this, for consistency with alien.buffer and
zile.Array.
Adjust all callers.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
fixup
syntax: use zile.Array for color attribute stacks.

* lib/zile/syntax.lua (state.colors, state.highlight): Use
zile.Array instead of general lib.stacks here, because we dealing
entirely with integers.

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