feat!: require send as well as sync

[ctrlaltf24]

When the lazy_static feature "spin_no_std" is enabled, the bounds of Send is required This may happen if another dependency adds that feature, then cargo unifies the two dependency trees erroneously, causing a compile error requiring Send on Runtime.

This is technically a breaking change in case anyone had a Sync but not Send type implementing Function. May want to consider a minor version bump.

See more: https://github.com/rust-lang-nursery/lazy-static.rs/issues/204

[cetra3]

I'm OK with this but would definitely make it a breaking change, which is not a bad thing!

[partim]

Hello!

Is there a chance that this PR could get merged soon and a release made? The underlying issue is blocking us in Krill from upgrading some dependencies leading to quite a lot of duplicate library crates being used slowing down compilation and bloating the binary.