pb2py: FIELDS moved to classmethod get_fields()

[ph4r05]

• https://github.com/trezor/trezor-common/issues/171
• garbage collectable after (de)serialization
• memory savings are considerable, worth merging
• after the merge a minor update to protobuf.py is needed

[ph4r05]

It is also possible to import on demand. Ideally to use on-demand imports only for micropython code.

# Automatically generated by pb2py
# fmt: off
import protobuf as p

class MoneroOutputEntry(p.MessageType):

    def __init__(
        self,
        idx: int = None,
        key: MoneroRctKey = None,  # non-micropython interpreter may be annoyed by this
    ) -> None:
        self.idx = idx
        self.key = key

    @classmethod
    def get_fields(cls):
        from .MoneroRctKey import MoneroRctKey  # change here
        return {
            1: ('idx', p.UVarintType, 0),
            2: ('key', MoneroRctKey, 0),
        }

[jpochyla]

Lets postpone the deferred imports for now, merging!