Improve performance of A6 algorithm and input code

[ivan-shrimp]

This fix involves:

• Simplifying the algorithm for Problem A6,
  removing the need for a custom right_exp_search routine.
• Creating the a6_benchgen crate, containing:
  • a library with an algorithm that generates bad-case data for A6; and
  • a command line tool that outputs said data with some customization options.
• Adding a zero-copy parsing method to read_u32.
• Changing the API of read_u32 to allow U32Reader to own an underlying reader,
  and removing the read_line method which is incompatible with this change.
  • This involves changes in both A5 and A6.
• Removing all uses of unstable features.
  • The fact that U32Reader can own a static lock to stdin depends on Make regular stdio lock() return 'static handles rust-lang/rust#93965,
    which is stabilized in Rust 1.61, but hasn't made it to stable yet.

[ivan-shrimp]

Looks like the comment above caused GitHub to say "ivan-shrimp mentioned this pull request" in the Rust PR mentioned above.

I'm a student, this is just a small programming competition project and the mention was unintended. If you're a Rust developer that came across this due to the mention, I'm really sorry and I'd like to thank you for improving Rust and allowing all of us to write correct and maintainable code.