Add 'list' subcommand for palette, imageset, sprite, tilemap and bitmap elements

[CJLove]

Just getting my feet wet with Rust :). This PR adds a list subcommand for the above elements so the user running aloevera interactively on a project file can see a summary of what elements exist.

Sample output (logging elided):

> aloevera -p /tmp/test.av palette list
Palettes:
  kq5_pal: 256 colors

> aloevera -p /tmp/test.av bitmap list
Bitmaps:
  kq5_bmp: width 320 depth 8

> aloevera -p /tmp/test.av imageset list
Image sets:
  kq5_screen: 1 320x100 frames depth 8

> aloevera -p /tmp/chopper.av sprite list
Sprites:
  chopper_sprite: 64x32 depth 4

> aloevera -p /tmp/chopper.av tilemap list
Tilemaps:
  chopper_tilemap: map 128x64 tiles 8x8 mode Tile4BPP

[yeastplume]

Looks great, especially if it's your first time out with Rust!

It might be an idea to have a list 'all' command as well that outputs all elements in the file. The current data is just dumbly serialized to a very unnecessarily chunky json format, and I think addressing #2 will involve writing a custom serde binary serialiser which will mean we'll need to have commands to output the contents of a project file. So very handy to have this in place before that work is done.