Relax SparseMap's Copy constraint, change `SparseMap::remove`, add examples

[Chriscbr]

The Copy constraint on values seemed like it was necessary at some point, but at least from what I can see, there's no hard requirement on it. The only possible limitation is that SparseMap::update needs to accept a Fn(&V) -> V instead of Fn(V) -> V since it's not possible to safely map and replace a value at the same time (https://stackoverflow.com/questions/67461269/replace-a-value-behind-a-mutable-reference-by-moving-and-mapping-the-original).

Breaking changes:

• SparseMap::remove now returns Option instead of bool.
• SparseMap::update now accepts f: F where F: Fn(&V) -> V instead of F: Fn(V) -> V.

[Chriscbr]

(I should break some of these changes into separate PRs, don't follow my coding practices too closely...)