curly braces {} are not url-safe

the main alternative for object-unions was round parens () which are url-safe, but i was worried they'd be more likely to cause problems with URL detection -- markdown uses () as part of link notation for example.

i'm not actually sure how tolerant URLs are of literal curlies -- maybe it's mostly fine?

it's also not like super-critical that the syntax is all URL-allowed, worst case it just needs to be encoded (which it should be anyway)