Closes #367.

* Implement MallocSizeOf for SmallVec (v2)

Signed-off-by: Nico Burns <[email protected]>

* Bump malloc_size_of crate to version with lower MSRV

Signed-off-by: Nico Burns <[email protected]>

---------

Signed-off-by: Nico Burns <[email protected]>

* v2: Bump MSRV to 1.83

* Run GitHub checks on merge_group

* Update the `extract_if` feature

* doctest

Currently `smallvec![]` expands to this:
```
{
    let count = 0usize;
    #[allow(unused_mut)]
    let mut vec = ::smallvec::SmallVec::new();
    if count <= vec.capacity() {
	vec
    } else {
	::smallvec::SmallVec::from_vec(::alloc::vec::Vec::new())
    }
};
```
This commit adds a rule to the `smallvec!` macro for the zero-length
case so it instead expands to this:
```
::smallvec::SmallVec::new()
```
The `std::vec!` macro already has a similar special case.

This commit also improves the non-zero case.
- It removes the `#[allow(unused_mut)]`, which was only needed for the
  zero-length case.
- It changes the `*` repetitions to `+`. (Again, like `std::vec`.)