Run less filter on main fiber

[denisdefreyne]

This ensures that Less is always invoked on the main fiber. Potential fix/workaround for #999.

• Added Filter#on_main_fiber, which executes the given block on the main fiber. This is the main mechanism that makes the Less filter work again.

• Converted all Less tests to specs. These are high-level tests (compiling using the CLI) because the Less filter is somewhat annoying to test otherwise (it now requires interaction with the compiler).

• Two new Less specs that did not exist before are skipped for now, as I can’t get them to work. Will investigate in another PR.

[denisdefreyne]

less_spec.rb now passes, but test_less.rb fails, with two distinct reasons:

• Less::Error: Maximum call stack size exceeded
• FiberError: dead fiber called

[denisdefreyne]

False hope: less_spec.rb fails too, sometimes.

[denisdefreyne]

Not quite working. I reported an issue to therubyracer: rubyjs/therubyracer#421.

[denisdefreyne]

Added a mechanism to make code execute on the main fiber, and made the less filter use it. This seems to work!

[denisdefreyne]

CC @RubenVerborgh — can you verify that this PR fixes your problem (#999)?

[denisdefreyne]

Will merge because I cannot reproduce the problem on Travis CI nor locally on a Debian VM.

[RubenVerborgh]

That was fast 😉 I'll test later today.

[denisdefreyne]

The two new skipped Less specs are implemented in #1003.

[RubenVerborgh]

Looks good to me.