remove the ability to provide a slice of function constructors

I don't think it is ever useful

fix some bugs around providing slices []T

allow checking if a Provide is Nil

add support for variadics

check if provider is explicitly set

rename interface types and add isSet

Add explicit Optional types

s/interface{}/any

Add contributing section

Update README

Doc rewriting

Refactor tests

Add New Example

Add example

Initial Version