Implement reserve_len and reserve_len_exact for VecMap

[csouth3]

Implement reserve_len and reserve_len_exact for VecMap in accordance with rust-lang/rfcs#509.

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[csouth3]

r? @gankro

[Gankra]

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.

[csouth3]

So then would the correct wording for VecMap be to change the first sentence to
"Reserves capacity for the given VecMap to contain len distinct elements..." and then keep the rest of what I had, or should the second sentence be changed to mirror BitvSet too? Can't quite tell what would make sense

[Gankra]

Yeah s/BitvSet/VecMap; they have basically the same semantics (the high-level documentation even suggests that BitvSet is basically VecSet).

[Gankra]

Only issue is the method description bikeshed.

[Gankra]

Also for posterity, exact:

/// Reserves the minimum 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`.
///
/// Note that the allocator may give the collection more space than it requests. Therefore
/// capacity can not be relied upon to be precisely minimal. Prefer `reserve_len` if future
/// insertions are expected.

[csouth3]

Method descriptions fixed.

[csouth3]

Alright, take 3!