Added method to use to existing libuv instance

[alshdavid]

Hi all, this PR adds a method to construct a Loop from an existing pointer address.

My use case is reusing the Libuv bindings you have with an existing libuv instance - specifically, I am writing a Nodejs native addon and want to reuse the bindings offered by this package.

Used here from my fork: https://github.com/napi-rs/napi-rs/pull/2154/files#diff-1e236034b744d0ce6068c6b239c36a821d5550789289a8715c70c61c7ff9bbadR56

Testing this locally with a Nodejs add on works as expected