Update to 2021 edition, bless tests

[nyurik]

• Move to edition 2021, and remove redundant use in test
• run tests with TRYBUILD=overwrite

[tatsuya6502]

Thank you very much for submitting a PR!

I believe we can ignore the failing tests:

• CI / test (nightly)
  • It runs with cargo update -Z minimal-versions, and is already failing on the main branch in a few weeks.
• CI (Quanta disabled) / test (nightly)
  • Same to above
• Linux cross compile tests / linux-cross (mips-unknown, ...)
  • Due to bugs in rustc 1.73: With Rust 1.73.0, some unit tests started to fail for mips-unknown-linux-musl target #333
• Linux cross compile tests / linux-cross (mipsel-unknown, ...)
  • Same to above

[nyurik]

see #341 -- it should fix this

[tatsuya6502]

Thank you. It looks good to me.