Helper function to create, but not save, related entities

[MordantWastrel]

Problem: .create() always persists entities and while you used to be able to use associate() on a belongsTo() to associate objects in memory without saving them, now you can't do that with new entities since you can't call Posts.user() unless Post has been hydrated.

Proposed solutions:

1. Create a .new() function that does exactly what .create() does, only doesn't save anything; or
2. Add a save argument to .create() that defaults to true, but when false, skips the .save() call.