Remove leading spaces after linebreak

[mikeando]

There are three independent commits

The initial commit just tidies up parse_inline a little by:

• assigning directly to new_inlas the return value from the match rather than inside each branch.
• moving some if c=='X' && something() {...} conditions inside the default match arm into their own arms.

The second commit change the way hard linebreaks were rendered in the CM output from '␣␣\n' to '\\n' to allow
for round-tripping when the LineBreak was on an otherwise empty line.

The final commit just left-trims the content in parse_inline if the last node was a LineBreak.
This removes the extra space character that was found in #213.

I'm not 100% sure this is the right way to fix it, but all the cargo test tests pass, and the spec tests all parse.

[kivikakk]

Code changes are good, tested it out a bit locally, looks great — thanks!