update required fields automatically when fields have default values

[loyada]

In the following examples

class Foo(Structure):
   i: int = 5
   s1: String = "x"
   arr: Array(default=[1,2]) 
   map: dict
   _required=['arr', 's1', 'i']

"_required" should be automatically updated not to include any field that has a default value, so that the following:
Foo(map={})

is value.

Also, if no "_required" attribute is defined (which implies that all the fields are required), one will be created, and will contain only the fields that have no default values.

[loyada]

done in 3248206