* Add license specifier to all Cargo.toml files
Explicitly specify `license = "GPL-2.0-only"` in each Cargo.toml
file. The wording is from the SPX guidelines.
* Add shell script to check licenses of Rust projects
rust/check_licenses.sh automatically verifies all dependencies
for accidental usage of GPLv3. Since basically nobody in the
universe uses GPLv3, it tends not to do much.
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