csp.Struct needs to deepcopy its default values

[robambalu]

Describe the bug
If a csp.Struct type is defined with a mutable default ( ie dict ), then all instances of that type will share the same default. Mutate it in one, will mutate for all. This is similar to the age-old python mutable default argument problem, but we have the ability to fix it by deepcopying defaults instead of shallow copying

To Reproduce

class S(csp.Struct):
    d : dict = {}

s1 = S()
s2 = S()

s2.d['a'] = 1
print(s1)
> S( d={'a': 1} )