Add tokio taskname registration for use in tokio-console #626

Job	Run time
Documentation	18s
Lints (--all-features, RUSTFLAGS="--cfg tokio_unstable")	46s
Lints	51s
Lints (--no-default-features)	44s
Minimum Supported Rust Version	25s
Minimal Dependency Versions (MSRV)	34s
Minimal Dependency Versions	50s
Build (aarch64-unknown-linux-gnu, --all-features, RUSTFLAGS="--cfg tokio_unstable")	3m 36s
Semantic Versioning	24s
Build (aarch64-unknown-linux-gnu)	3m 18s
Test Suite (--all-features, RUSTFLAGS="--cfg tokio_unstable")	4m 2s
Build (aarch64-unknown-linux-gnu, --no-default-features)	3m 22s
Memory leaks	3m 45s
Build (x86_64-pc-windows-gnu, --all-features, RUSTFLAGS="--cfg tokio_unstable")	3m 50s
Test Suite	4m 0s
Build (x86_64-pc-windows-gnu)	3m 41s
Test Suite (--no-default-features)	3m 57s
Build (x86_64-pc-windows-gnu, --no-default-features)	3m 40s
Build (x86_64-unknown-linux-gnu, --all-features, RUSTFLAGS="--cfg tokio_unstable")	3m 41s
Build (x86_64-unknown-linux-gnu)	3m 5s
Build (x86_64-unknown-linux-gnu, --no-default-features)	3m 7s
Publish version	0s
	51m 56s

Provide feedback