In the Python generator, create aliases for numeric types

When users are consuming generated code in Python, they are not able to tell which integer type a given `int` was or what size a float was. We can make this somewhat easier by introducing aliases for used numeric types that preserve that type info.

e.g.
```py
def foo(a: int) -> int:
   ...
```
becomes
```py
s32 = int
s64 = int

def foo(a: s32) -> s64:
   ...
```