chore: fix many clippy warnings

[Standing-Man]

• fix many clippy warnings

warning: `mini-lsm` (lib test) generated 16 warnings (11 duplicates) (run `cargo clippy --fix --lib -p mini-lsm --tests` to apply 5 suggestions)
warning: hiding a lifetime that's elided elsewhere is confusing
  --> mini-lsm-mvcc/src/tests/harness.rs:76:12
   |
76 |     fn key(&self) -> KeySlice {
   |            ^^^^^     ^^^^^^^^ the same lifetime is hidden here
   |            |
   |            the lifetime is elided here
   |
   = help: the same lifetime is referred to in inconsistent ways, making the signature confusing
help: use `'_` for type paths
   |
76 |     fn key(&self) -> KeySlice<'_> {
   |                              ++++

warning: `mini-lsm-mvcc` (lib test) generated 17 warnings (12 duplicates) (run `cargo clippy --fix --lib -p mini-lsm-mvcc --tests` to apply 5 suggestions)

[skyzh]

thanks! time to bump my rust compiler..

[skyzh]

ty!