Update proc-macro to fix build

[HaoranYi]

It looks like cargo build is not happy with proc-macro2 1.0.56 any more.

error[E0422]: cannot find struct, variant or union type `LineColumn` in crate `proc_macro`
   --> /home/sol/.cargo/registry/src/index.crates.io-6f17d22bba15001f/proc-macro2-1.0.56/src/wrapper.rs:475:33
    |
475 |                 let proc_macro::LineColumn { line, column } = s.start();
    |                                 ^^^^^^^^^^ not found in `proc_macro`
    |
help: consider importing this struct through its public re-export

Update to proc-macro2 1.0.78 to fix the build.

[mvines]

Strange, builds fine for me even without this PR. But if bumping proc-macro helps 4u, that's good enough for me. Thanks for the fix