color-array: subtype of color

Rather than having the color class handle both 'spot' colors and arrays
of colors (like pixel buffers), we split the latter functionality out to
a new class, color-array.
encoding, colorspace, color, color-array: prefix record type names with ':'

To avoid polluting the namespace with bindings of common words like
"color", we follow the convention shown in srfi-99 of prefixing the
record type names with a ":".  The rtds ":color" and ":color-array" are
exported by default, and we may do the same for the others later, if
needed.