The key parameter of the insert functions are of type String now.
As insert requires ownership of its parameter `key` and StrAllocating
being gone in rust these functions now communicate that ownership
in their signature.
As a result &str must be converted into a String at the caller instead
of the callee.
The Trait ToOwned isn't an alternative as it's a generalized Clone.
StrAllocating simply has hidden an allocation or move.