Add more Vec/slice-like methods to maps and sets

[cuviper]

impl<K, V, S> IndexMap<K, V, S> {
    pub fn truncate(&mut self, len: usize);
    pub fn split_off(&mut self, at: usize) -> Self where S: Clone;
    pub fn first(&self) -> Option<(&K, &V)>;
    pub fn first_mut(&mut self) -> Option<(&mut K, &mut V)>;
    pub fn last(&self) -> Option<(&K, &V)>;
    pub fn last_mut(&mut self) -> Option<(&mut K, &mut V)>;
    pub fn swap_indices(&mut self, a: usize, b: usize);
}

impl<T, S> IndexSet<T, S> {
    pub fn truncate(&mut self, len: usize);
    pub fn split_off(&mut self, at: usize) -> Self where S: Clone;
    pub fn first(&self) -> Option<&T>;
    pub fn last(&self) -> Option<&T>;
    pub fn swap_indices(&mut self, a: usize, b: usize);
}

I would prefer &K instead of &mut K in IndexMap::first_mut and
last_mut, but this is consistent with the existing get_index_mut.

[bluss]

get_index_mut seems to be an oversight. Mutable key access is supposedly hidden away in the trait MutableKeys and I think it seems ok to continue that.

[bluss]

If you agree I'd do &K here, that's the initial interface we were going for in this crate (while still allowing opt-in access to mutable keys with the trait). Implementing a such "first_mut2" in MutableKeys could be done on a YAGNI basis 🙂

[bluss]

Comment about &K applies

[bluss]

I guess I technically can't approve while there are still methods without doc comment, but I like the idea.

[cuviper]

OK, I added more docs for the public methods.

At some point we should probably go for #![deny(missing_docs)], because there are a lot more missing, but I don't want to force that into this PR.

[cuviper]

Oh, I switched to &K too -- force-pushed to split up that commit.