Hug lambda arguments in google-java-format.

[copybara-service]

Hug lambda arguments in google-java-format.

When a call doesn't fit on one line and its only argument is a lambda, keep the
lambda's head on the call's line and indent its body relative to that line:

executor.submit(() -> {
  run();
});

instead of

executor.submit(
    () -> {
      run();
    });

The lambda is hugged only if everything up to -> fits on the line; otherwise
the existing layout is used. However, if breaking before the lambda allows the
entire lambda to fit flat on the next line, prefer breaking before the lambda
rather than hugging params -> onto the call line and breaking inside the
lambda body. Lambdas preceded by other arguments are not hugged, since the
preceding arguments wouldn't line up with the lambda's body. This addresses
#19.

This adds a Doc.Choice node to the layout IR, which chooses between a
preferred and a fallback layout with the same flat text. LayoutBuilder
derives the preferred layout by flattening the argument list up to the hug
point.