Add conversion to SystemTime within ReactionCtx

[oowekyala]

So with @AneesHl and @cmnrd we talked about replacing the use of Instant with SystemTime within the runtime. One problem of Instant is it is opaque, although it can easily be turned into a Duration (from program startup). With SystemTime we would have a Duration since the Unix Epoch, which helps in the short term with LF-Rust/LF-C interop, and in the long term, maybe with federated execution.

Instead of replacing Instant completely though, I wonder if the following changeset would also do the trick. This just adds a conversion function from Instant to SystemTime using a reference point taken at the startup of the program. The runtime otherwise still uses Instant internally, which is nice because it is monotonic.

[cmnrd]

Looks good! But it looks like clippy isn't happy with the change.