Update the syn crate and adopt let_else in three more places

Update the syn crate and adopt let_else in three more places

The syn crate has gained support for let_else syntax in version 1.0.76,
see https://github.com/dtolnay/syn/pull/1057 .

In the three instances that use let_else, we've sent code through an
attr macro, which would create compile errors when there was no
let_else support in syn. To avoid this, we ran
`cargo +nightly update -p syn` for updating the syn crate.