Use save-image to write a new executable

[larsbrinkhoff]

Use the save-image to write a new executable with everything pre-loaded.

• It will make Forth start faster.
• It will test the save-image library.
• There will be an incentive to make save-image work in more targets.

[pipcet]

I'm not sure how much of a win it would be for asmjs (all data has to be encoded as javascript and go through the parser), but wasm would definitely benefit.

[larsbrinkhoff]

You are right, not all targets can or should support saving the image.