Implementing a UI element for viewing the 'examine' info

For now it just says what tile is under the cursor.
Adding json_arrlen()

To get the length of an array
Adding parse_color

Takes a string and returns the color, uppercase or lowercase

It will return garbage for strings that don't have the name of a color.
Adding gfx_srcjson

Takes a json_ref and returns a struct gfx
Prevent re-declaration of null in stringset enums
Improving tile implementation

Now called RL_Tile, they are organized into tilesets which can be
sourced from JSON. The structure of it still doesn't sit right with me
but it works for now, so I'll put off streamlining the interface for a
little bit.
Huge streamlining of the color system

I think this might be as small as a curses color handler can get, and
it's as intuitive as painting to use.
Proper implementation of RL_Ref_GetObjectsAt()

And moving everything over to the new color system