Switch to SPDX 2.1 license expression

Switch to SPDX 2.1 license expression

According to the Cargo Reference:
https://doc.rust-lang.org/cargo/reference/manifest.html

> This is an SPDX 2.1 license expression for this package. Currently
> crates.io will validate the license provided against a whitelist of
> known license and exception identifiers from the SPDX license list
> 2.4. Parentheses are not currently supported.
>
> Multiple licenses can be separated with a `/`, although that usage
> is deprecated. Instead, use a license expression with AND and OR
> operators to get more explicit semantics.