Make util/set-canvas-size! take map with w and h
Only create instrument when needed
Separate basic instrument from synths

* Allows creation of instrument that has basic piano-keys but not attached synths.
* Add add-synths-to-instrument which will take instrument and add synths.
* All this is to allow creation of instrument that can be drawn before synths have been created.  This lets us draw a realistic start screen without breaking sound in the browser.
Draw start screen that looks like actual instrument
Position instrument correctly
Re-enable sound
Correct indentation