Merge #54

54: Raw entry hasher r=Amanieu a=sujayakar

This PR allows the user to create a `HashMap` where `K` doesn't implement `Hash`, so long as they are okay with only using the raw entry API.

The first step is to move around some methods that didn't actually depend on `K: Hash`.  Then, the only non-plumbing change was to defer the `.reserve(1)` call on `raw_entry_mut` to right before insertion.  Then, we expose a new method `RawVacantEntryMut::insert_with_hasher` that takes in the custom hashing function and passes it in to `reserve` before inserting.

@Amanieu do these API changes merit a version bump?  I think we're only allowing these methods to be used on _more_ types, but let me know.

Closes #44 

Co-authored-by: Sujay Jayakar <[email protected]>