Add convenience functions to TypedStruct

[idanarye]

Assuming:

class Foo(TypedStruct):
    a = int
    a.default = 1

    b = str
    b.default = 'two'

We want the following functions:

• Setting a field to DEFAULT should use the default as defined in the typed struct:

  assert Foo(a=DEFAULT, b='four') == Foo(a=1, b='four')
  foo = Foo(a=3, b='four')
  foo.b = DEFAULT
  assert foo == Foo(a=3, b='two')

• .copy():

  foo = Foo(a=3, b='four')
  assert foo.copy() == foo
  assert foo.copy() is not foo

  Current behavior: foo.copy() returns a dict.
• .update():

  foo = Foo(a=3, b='four')
  foo.update(a=5)
  assert foo == Foo(a=5, b='four')
  with pytest.raises():
      foo.update(a='six')

  Current behavior: update is dict.update which bypasses the verifications.
• .but_with():

  foo = Foo(a=3, b='four')
  assert foo.but_with(a=5) == Foo(a=5, b='four')

• Setting a field to AUTO should leave it unchanged:

  foo = Foo(a=3, b='four')
  foo.b = AUTO
  assert foo == Foo(a=3, b='four')

  This will be useful with update() and but_with():

  def make_foo(a=AUTO, b=AUTO):
      foo = Foo(a=3, b='four')
      foo.update(a=a, b=b)
      return foo
  
  assert make_foo() == Foo(a=3, b='four')
  assert make_foo(a=5) == Foo(a=5, b='four')
  assert make_foo(b='six') == Foo(a=3, b='six')