Fix `cargo clippy`'s `warnings` and `errors`, and do `cargo fmt --all`

[eval-exec]

• Apply cargo clippy --fix --all
• **Fix cargo clippy errors:

❯ cargo clippy --all
warning: `panic` setting is ignored for `test` profile
    Checking sled v1.0.0-alpha.124 (/home/exec/Projects/github.com/spacejam/sled)
error: calling `set_len()` immediately after reserving a buffer creates uninitialized values
   --> src/heap.rs:516:9
    |
516 |         let mut data = Vec::with_capacity(self.slot_size);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
517 |         unsafe {
518 |             data.set_len(self.slot_size);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: initialize the buffer or wrap the content in `MaybeUninit`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec
    = note: `#[deny(clippy::uninit_vec)]` on by default

error: calling `set_len()` immediately after reserving a buffer creates uninitialized values
   --> src/metadata_store.rs:526:5
    |
526 |     reusable_frame_buffer.reserve(len + 12);
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
527 |     unsafe {
528 |         reusable_frame_buffer.set_len(len + 12);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: initialize the buffer or wrap the content in `MaybeUninit`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec

error: calling `set_len()` immediately after reserving a buffer creates uninitialized values
   --> src/metadata_store.rs:592:9
    |
592 |         low_key_buf.reserve(low_key_len);
    |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
593 |         unsafe {
594 |             low_key_buf.set_len(low_key_len);
    |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = help: initialize the buffer or wrap the content in `MaybeUninit`
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#uninit_vec

error: could not compile `sled` (lib) due to 3 previous errors