Reduce bloat from `smallvec!` in the inline case

[bolshoytoster]

Currently, smallvec!'s inline case is handled by repeatedly pushing to a SmallVec. I've measured that this produces a lot of LLVM IR + bloat in the output binary.

I've managed to reduce the size in the inline case by using From<[T, M]>:

($($x:expr),+$(,)?) => ({
    $crate::SmallVec::from([$($x,)+])
});

(Note: for that to have an effect, From<[T, M]> needs to be marked #[inline] as well)

I haven't measured how this affects the heap case, but if it's a concern, I've found that something like this reduces the output to the same size, but is a little more verbose:

($($x:expr),+$(,)?) => ({
    const COUNT: usize = 0usize $(+ $crate::smallvec!(@one $x))+;
    let vec = $crate::SmallVec::new();
    if false {
        vec
    } else if COUNT <= vec.capacity() {
        core::mem::forget(vec);
        $crate::SmallVec::from_buf([$($x,)+])
    } else {
        core::mem::forget(vec);
        $crate::SmallVec::from_vec($crate::alloc::vec![$($x,)+])
    }
});