Fails to build on nightly-2020-10-29 due to rustc regression

[dtolnay]

rust-lang/rust#78507 affects this crate.

error: unmatched angle bracket
   --> derive/src/lib.rs:163:72
    |
163 | fn decide_trivial(fields: &Fields) -> Result<fn(&Field) -> Result<bool>> {
    |                                                                        ^^ help: remove extra angle bracket

error: expected one of `,` or `>`, found `{`
   --> derive/src/lib.rs:163:74
    |
163 | fn decide_trivial(fields: &Fields) -> Result<fn(&Field) -> Result<bool>> {
    |                                                                          ^ expected one of `,` or `>`

[estebank]

This will be fixed in rust-lang/rust#78523