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.
The notation with slashes is deprecated in favor of explicit AND or OR.
As I understand it, Rust's license is MIT *OR* Apache-2.0 matching the meaning of *OR* defined by [SPDX Specification 2.1](https://spdx.org/spdx-specification-21-web-version):
> If presented with a choice between two or more licenses, use the disjunctive binary "OR" operator to construct a new license expression, where both the left and right operands are valid license expression values.
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