Add `splice`

[nnethercote]

ThinVec currently lacks the splice method. In rust-lang/rust#104754 I converted a bunch of Vec instances in rustc to ThinVec for some performance wins. I had to change one use of splice like this:

-                alternatives.splice(idx..=idx, inner);
+ 
+                // Equivalent to `alternatives.splice(idx..=idx, inner);`.
+                alternatives.remove(idx);
+                for p in inner.into_iter().rev() {
+                    alternatives.insert(idx, p);
+                }

which could cause quadratic behaviour.

[nnethercote]

There is even a TODO: implement splice? comment in the code.

[Gankra]

#41

[marcospb19]

Hey 👋, should this be closed due to #41?