Add davidtwco and adamgemmell to libs-contributors

[Amanieu]

Welcome to our new stdarch maintainers!

cc @davidtwco @adamgemmell

[github-actions]

Dry-run check results

[WARN  rust_team::sync] sync-team is running in dry mode, no changes will be applied.
[INFO  rust_team::sync] synchronizing crates-io
[INFO  rust_team::sync] synchronizing github
[INFO  rust_team::sync] 💻 Team Diffs:
    📝 Editing team 'rust-lang/all':
      Adding member 'adamgemmell' with member role
    📝 Editing team 'rust-lang/libs-contributors':
      Adding member 'adamgemmell' with member role
      Adding member 'davidtwco' with member role
    📝 Editing team 'rust-lang-nursery/libs-contributors':
      Adding member 'adamgemmell' with member role
      Adding member 'davidtwco' with member role