Implement `reserve_len` and `reserve_len_exact` for VecMap

Implement `reserve_len` and `reserve_len_exact` for `VecMap` in accordance with rust-lang/rfcs#509.
I settled on the following wording in BitvSet:

```
/// Reserves capacity for the given `BitvSet` to contain `len` distinct elements. In the case
/// of `BitvSet` this means reallocations will not occur as long as all inserted elements
/// are less than `len`.
///
/// The collection may reserve more space to avoid frequent reallocations.
```

I worded this to clarify "why len" and future-proof for a possible future where Vec and RingBuf get it. Willing to bikeshed on it, though.