Misc. inline box layout fixes

[nicoburns]

See individual commits:

• Fix whitespace collapsing when first item in a span is an inline box
  • <p><img src="..." /> label</p> should have a space between the image and "label".
• Fix wrapping when layout contains only inline boxes
• Test case for layout with only inline boxes
• Prevent inline boxes from being duplicated when the line ends with an inline box.
• Fix trailing whitespace computation:
  • If the last item on a line is an inline box, then it does not have any trailing whitespace.

[DJMcNab]

Is there something specific in here you want me to review - just want to check why my review has been requested?
Unfortunately, this code isn't something I have any real understanding of

[nicoburns]

Is there something specific in here you want me to review - just want to check why my review has been requested? Unfortunately, this code isn't something I have any real understanding of

Not really. Just keen to get it reviewed so we can get it merged (I think this is the only thing blocking a 0.3 release). I thought you had reviewed similar PRs recently, and might be up for reviewing this one. But no worries if not!

[DJMcNab]

This can only impact users of the tree style builder and/or inline boxes, both of which I think you're the only user of currently. So since this is unlikely to be swiftly reviewed otherwise, I'm happy to unblock.

Ideally, this would get a review from someone with domain knowledge at some point...