Add openable workbenches.

Refs #409.
inventory/slots: Remove duplicated code.
tools/noiseview: Switch to PNM.

Lets us take advantage of PyPy by dropping PIL.
tools: Example false-color noise viewing.

Looks like the number of octaves don't cause much corrugation, so maybe no
octaves are necessary.
tools/noiseview: Add switch for enabling color.
tools/noiseview: Clip noise.

Yeah, one all-too-easily forgets that octaves in Bravo have a theoretical
range far beyond their normal limits.
tools/noiseview: More color mapping.

Hm. The distributions are different from what I had expected/remembered. More
spading is required.
tools: Add tool to examine statistics in noise.
tools/noiseview: Use an inner slice for mushroom islands.

Just as an example.