Add `SmolStrBuilder` / `SmolStrBuffer`

[alexheretic]

To build a SmolStr from parts it would be useful to have a writeable builder.

Perhaps something like

let mut result = SmolStrBuffer::new();
for part in parts {
    result.push_str(part);
}
let result = SmolStr::from(result);

This almost already exists provided by smol_str::Writer, but using this has a couple of downsides

• Writer is hidden from docs, implying it isn't covered by stability guarantees.
• Writer doesn't provide infallible push_str, though it trivially could.

It seems straightforward to adapt Writer to this end.

[Veykril]

Writer was added as an implementation detail for the format_smolstr! macro, but we certainly could expose this with a nice API.