Small prerequisites for porting the Python EventBase hierarchy to Rust:
- duration: make `from_milliseconds` const and add an `IntoPyObject` impl
for owned `SynapseDuration`, so the new Rust `Event.sticky_duration()`
can return one directly to Python.
- internal_metadata: rename `copy()` to `deep_copy()` (matching the new
naming used by the rest of the events module) and make `new()` callable
from sibling modules.
- json_object: expose `object` as a `pub` field and add a `get_field`
helper so the new Event class can read from it without going through
Python.
- signatures, unsigned: add `deep_copy()` methods so the new Event class
can implement its own deep-copy.
This changes the arguments in clock functions to be `Duration` and
converts call sites and constants into `Duration`. There are still some
more functions around that should be converted (e.g.
`timeout_deferred`), but we leave that to another PR.
We also changes `.as_secs()` to return a float, as the rounding broke
things subtly. The only reason to keep it (its the same as
`timedelta.total_seconds()`) is for symmetry with `as_millis()`.
Follows on from https://github.com/element-hq/synapse/pull/19223
Erik Johnston