Rust 2021 Macro Warning Cleanup

Small warning message cleanup of warnings related to upcoming changes in Rust. e.g:

warning: trailing semicolon in macro used in expression position
   --> api/src/web.rs:142:4
    |
142 |         };
    |          ^
    |
   ::: api/src/handlers/blocks_api.rs:203:18
    |
203 |         let el = right_path_element!(req);
    |                  ------------------------ in this macro invocation
    |
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #79813 <https://github.com/rust-lang/rust/issues/79813>
    = note: this warning originates in the macro `right_path_element` (in Nightly builds, run with -Z macro-backtrace for more info)

Also cleans up unused import warnings from cargo tests.

mimblewimble / grin

Rust 2021 Macro Warning Cleanup #3658