Eliminate .next-line; add .instructions div to show the code again, with an arrow pointing to current instruction.
Auto-scroll the .instructions div to show the next one with animation
Get rid of remaining references to .next-line CSS
Bug fix: line number gutter background was red for gold screen
Bug fix: resizing when code was showing in machine would add lots of padding to every line because I reused .code class for two different things
Eliminate width fudging by switching to border-box box layout model
Use getBoundingClientRect to avoid hard-coding sizes of divs.  Also make .actions position:fixed in case we need to scroll.
Set width to 50% so recalculation is faster
Bug fix: couldn't set heights if there was no .actions div.
Add step button
Print console output from bottom of window and automatically scroll so bottom is always in view
Label above Instructions and Input&Output divs
Print output from the top, not the bottom, for simplicity of programming and learner understanding.
Extra padding around .code-editor and .machine if width is at least 1000px
Simple compiler for Ruby puts statements