Running swift-format over the Pigeon-generated output turned up a bunch of places where our generator output doesn't match formatted output. While we don't need it to be perfectly formatted, we do generally want to match expected style in the initial generation. And in a number of cases, the output is distinctly non-idiomatic, like parens in if statements.

Some things to fix based on initial skim (but when doing this we should iterate a bit with the autoformatter and try to fix anything that's relatively straightforward):

• if statements should not use ()s.
• cases should not be indented relative to the switch
• Conditional imports should be indented
• Multi-line arrays should use trailing commas
• Named params should have a space after :, which is missing in some places