Extract imports out from `__init__()` and `__type_support__()`

[InvincibleRMC]

Description

For about a 5% speed up in message construction times and simplification of importing logic by moving imports out of __init__() and __type_support__() .

All part of a grand plan to maybe support Optional Idl types by further simplify the __init__() method.

Is this user-facing behavior change?

No only internals should change.

Did you use Generative AI?

Additional Information