Added helper function to immediately create an object fixture

[romrell4]

As discussed in https://github.com/appmattus/kotlinfixture/issues/98, it would be nice to have a small helper function that would immediately create a randomized fixture, without having to write:

kotlinFixture().invoke<MyType>()

each time.

This PR adds a new helper function to make ☝️ just look like:

fixture<MyType>()