Implement Span::get

[Alextopher]

Partially revives #456 and merge would partially close #455.

This implementation changes the name of the new feature in #456 from Span::sub_span to Span::get to have parity with str::get.

This implementation does not require std.

[tomtau]

this may still fail in no_std... if you'd like to test no_std, you can try to build it as follows: rustup run nightly -- cargo build -j1 -Z build-std=core,alloc --target x86_64-unknown-linux-gnu

[Alextopher]

I have never been in a no std environment before. Actually in general this is my first Rust OSS participation.

I couldn't get your suggestion to work but https://lib.rs/crates/cargo-no-std-check seemed to do the trick to test the build for no_std.

[tomtau]

Thank you and congratulations on your first Rust OSS participation! 🎉