feat(builder): 2-step building a facet

[riezebosch]

suggestion for a clean solution to infer the source type by splitting the setup into two steps, fixes #25

It also opens a nice API for bundling more methods on a builder instead of overloading all types with extension methods (like AwesomeAssertions vs Shouldly)

[Tim-Maes]

Few reasons I don't really like this:

Facet is doing transformations, not construction. Fundamentally different
Type inference as is now, always works. Builder doesnt' improve this.
This extension adds a ToFacet()... to every single type in the codebase
Every call allocates a Builder object just to call one method on it.

Going to close this PR now, sorry for the delayed response. Thanks

[riezebosch]

the ToFacet extension is already added by the original source code to every single type in the codebase, right? https://github.com/Tim-Maes/Facet/blob/master/src/Facet.Extensions/FacetExtensions.cs#L62