Update rental to 0.5.6

Your crate depends on an old version of `rental`, which relies on
a backwards-compatibiltiy hack in rustc. In future versions of rustc,
versions of `rental` below `0.5.6` will now compiling, breaking your
crate.

This PR updates your crate to `rental` 0.5.6, which ensures that it will
continue to compile with both older and newer version of rustc.

For more information, see rust-lang/rust#83125

Author: Aaron1011