add a type implementing `BorshSchema` of `SmallVec`

create a type that cannot be instantiated representing the schema for a serialized `SmallVec` with `borsh::BorshSchema`

`borsh` is a serialization framework

concept:

```rust
pub enum WhateverName {}

impl BorshSchema for WhateverName {/* ... */}
```

this should be added to the `borsh.rs` file