Add native to_dict method for structs

[arhamchopra]

This PR adds a new to_dict method for structs which is written fully in C++. The older python based to_dict method has been renamed to to_dict_depr.

Motivation: The newer C++ based to_dict method is faster than the original python based to_dict method. Some timing examples are shown below:

class A(csp.Struct):
  a: int
  b: str
  c: bool
  d: float
a = A(a=1, b="1", c=True, d=1.0)
%timeit a.to_dict()         # 0.888 microseconds
%timeit a.to_dict_depr()    # 4.98 microseconds

class E(csp.Struct):
  a: list
  b: dict
  c: tuple
  d: set
e = E(a=[1,2,3], b={1:1, 2:2, 3:3}, c=(1,2,3), d={1,2,3})
%timeit e.to_dict()        # 2.65 microseconds
%timeit e.to_dict_depr()   # 15.4 microseconds