Bump version to 0.2.70

[tblah]

#1745 is needed to fix the compilation of effective-limits on riscv64gc-unknown-linux-gnu. This will pave the way for RISC-V support in rustup.

The version bump will allow effective-limits to depend upon the fix.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @JohnTitor (or someone else) soon.

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[JohnTitor]

Released on crates.io, thanks!