Add ability to generate Builder struct

[amis92]

Builder would allow for easier instantiation in tree structures or multiple mutations (close to ImmutalbeList<T>.Builder), eg.

for:

partial class Person
{
  string FirstName { get; }
  Address PrimaryAddress { get; }
}

generator would provide, in addition to primary ctor, mutators and collection mutators, a builder:

partial class Person
{
  Builder ToBuilder() => new Builder { FirstName = FirstName, PrimaryAddress = PrimaryAddress };
  struct Builder
  {
    Builder(Person original)
    {
      FirstName = original.FirstName;
      PrimaryAddress = original.PrimaryAddress;
    }
    string FirstName { get; set; }
    Address PrimaryAddress { get; set; }
    Person ToRecord() => new Person(FirstName, PrimaryAddress);
  }
}

---

Usage

var person = GetVIP();
var newAddress = GetNewAddress();
var relocated = new Person.Builder(person) { PrimaryAddress = newAddress }.Build();

Of course the example above might easily be shortened as var relocated = person.WithPrimaryAddress(newAddress); but this is more useful when updating many properties at the same time.