support substrate 3.0.0. tested with paritytech/substrate@743accbe3256de2fc615adcaa3ab03ebdbbb4dbd
depend on substrate git without pinning a revision. use cargo to select a revision:
# may need to do twice!
cargo update -p sp-std --precise 743accbe3256de2fc615adcaa3ab03ebdbbb4dbd
cargo update -p sp-std --precise 743accbe3256de2fc615adcaa3ab03ebdbbb4dbd