Use SmallVec instead of Vec

### What is wrong?

There are places where we could be using [`SmallVec`](https://docs.rs/smallvec/1.6.1/smallvec/)s to reduce heap allocations.

For example, the function statement `Vec`s being returned in the [lowering pass](https://github.com/ethereum/fe/blob/master/compiler/src/lowering/mappers/functions.rs#L32) will always be pretty small.

We should update any other places where we are using small vectors, but not `SmallVec`.

### How can it be fixed

:keyboard:

Start with compiler/src/lowering/mappers/functions.rs, especially the function statement Vecs returned in the lowering pass. Then inspect the compiler for other small vectors that could use SmallVec; done means the applicable cases are updated without changing behavior.

Written by the indexing model from the issue text.