Various build fixes, including fixes to minimum version requirements

[g2p]

Tested with:

cargo +nightly update -Zminimal-versions
cargo check --all-features
cargo +nightly check --all-features --all-targets

[g2p]

Addresses #297. Ideally this would be tested in CI.

[g2p]

Build failure with rust 1.60 is unrelated; the following runs fine:

cargo +nightly update -Zminimal-versions
cargo +1.60 check --all-features

[JohnTitor]

Well, have you run the above comment on workspace?

[g2p]

Yes. The command in the first post is the motivation for this PR, it didn't run in master, with correctly specified dependencies (proc-macro2 and trybuild) it works.