Simplify attribute parsing

[brson]

There may be enough lookahead in the parser to simplify attribute parsing, which is quite intricate.

[marijnh]

Seeing the difference between #[foo] (without semicolon) and #[foo];, where foo can be any nested attribute expression, requires arbitrary lookahead. I assume that is what this issue is referring to. We've been actively avoiding arbitrary lookahead in other cases, so I'm closing this issue on the assumption that we don't want it here either.