Fix for #12055.

The fix itself is very simple. I've added tests for the original bug and also some other weird cases which were previously producing incorrect output.

I am assuming that existing behavior for standalone expressions is correct.

i.e. generate( t.objectExpression([]) ).code should produce {} not ({}). Certainly that's what I want it to be!

One case produces excess parentheses:

t.arrowFunctionExpression(
  [],
  t.sequenceExpression([
    t.objectExpression([]),
    t.numericLiteral(1)]
  )
)

This generates () => (({}), 1). I couldn't see a way to avoid this without introducing excess complexity for a pretty uncommon case.

Please shout if you want any changes.