Add append methods

[cuviper]

These are like a mix of BTreeMap::append / BTreeSet::append and Vec::append, updating existing entries or else appending new entries, while keeping the original order.

pub fn IndexMap<K, V, S>::append<S2>(&mut self, other: &mut IndexMap<K, V, S2>)
pub fn IndexSet<T, S>::append<S2>(&mut self, other: &mut IndexSet<T, S2>)

Closes #336.