Add get_mut_or_insert

[Yosi-Hezi]

This PR adds the get_mut_or_insert method, which is based on get_or_insert but returns a mutable reference instead of an immutable reference.
Relevant open issue: #30

[Yosi-Hezi]

There is a version of get_mut_or_insert that is based on #150 in case it will be accepted before this one.
I pushed this PR as well because it doesn't break the API, and I could really use the get_mut_or_insert functionality in my own code.

[Yosi-Hezi]

Maybe a better name would be get_or_insert_mut?

[jeromefroe]

@Yosi-Hezi do you want to rebase onto master now that #150 has been merged? My personal inclination would be for get_or_insert_mut as well :)

[Yosi-Hezi]

@jeromefroe rebased

[matklad]

Suggested change

	pub fn get_or_insert_mut<'a, F>(&mut self, k: K, f: F) -> &'a mut V
	pub fn get_or_insert_mut<'a, F>(&'a mut self, k: K, f: F) -> &'a mut V

#153

[Yosi-Hezi]

@matklad fixed.

[jeromefroe]

Thanks for the PR @Yosi-Hezi!