This commit implements a workaround for an [issue within rustc]. The
problem showed itself when using e.g. a `Vec<&str>` argument in an async
route handler (but not `&str`), which resulted in a "implementation of
`FromForm` is not general enough" error. The workaround itself works by
gathering all invocations of `FromForm`'s methods inside a block without
any `.await` points [ref].
[issue within rustc]: https://github.com/rust-lang/rust/issues/69663
[ref]: https://github.com/rust-lang/rust/issues/57478#issuecomment-501186084