Fix `let` keyword removal suggestion in structs (1.) Fixes a bug where, given this code: ```rust struct Foo { let x: i32, } ``` We were parsing the field name as `let` instead of `x`, which causes issues later on in the type-checking phase. (2.) Also, suggestions for `let: i32` as a field regressed, displaying this extra `help:` which is removed by this PR ``` help: remove the let, the `let` keyword is not allowed in struct field definitions | 2 - let: i32, 2 + : i32, ``` (3.) Makes the suggestion text a bit more succinct, since we don't need to re-explain that `let` is not allowed in this position (since it's in a note that follows). This causes the suggestion to render inline as well. cc `@gimbles,` this addresses a few nits I mentioned in your PR.