Add option to allow None on specific TypedStruct field

[idanarye]

TypedStruct fields are forcing a type, but sometimes we want to explicitly allow None values:

class Foo(ts.TypedStruct):
    bar = ts.Field(int, allow_none=True)

foo1 = Foo(bar=None)

Questions:

• Is allow_none a good name? Traditionally these types of fields would be called "optional", but this may be interpreted as "you don't need to supply a value"(which is what default= does), not necessarily as "you can put None in there"
• Should allow_none imply default=None?
• Do we want to limit this just for None values? It may be useful to also allow tokens. Maybe something like:

  class Foo(ts.TypedStruct)
      from easypy.tokens import AUTO, MAX, MIN
  
      bar = ts.Field(str, allow={None, AUTO})
      baz = ts.Field(int, allow={MIN, MAX})
  
  foo1 = Foo(bar=None, baz=MIN)
  foo2 = Foo(bar=AUTO, baz=MAX)

@koreno?

[koreno]

I'm not sure about the motivation.
It seems that if you're reading from some foreign source, and looking to create a TypedStruct from foreign data, there should be code that converts such None/MIN/MAX/AUTO into the appropriate types.
Or, Field should allow multiple types: ts.Field({str, NoneType, AUTO}). I don't think allow is very elegant...

[tigrawap]

I'd say - it as a road to everything a dict hell, make bunch great again.

typed should be typed, no optional types, if no value supplied(i.e kwargs unpacking from external source) and default is set - use default for initialising

Might also allow DEFAULT token, which should be used in typed functions definitions, which later chained to struct creation

This will indicate that default should be used. Plain None == exception