Bindings `Timestamp::elapsed`: point to `Span` interface in panic message

An appealing misuse of the module Timestamp interface is to use it to time reducer execution. It will not work for this; within a reducer invocation, Timestamp::now returns a constant value, the start of the transaction. This is intended behavior and should not change.

Timestamp::elapsed panics when attempting to mis-use that interface in this way. We should expand the panic message to encourage users to use time_span::Span instead.

E.g. we should tell users to rewrite this:

let now = Timestamp::now();
workload();
let elapsed = now.elapsed().as_secs();

into:

use spacetimedb::time_span::Span;

let span = Span::start("My workload");
workload();
span.end();

Locate the implementation of Timestamp::elapsed and its panic path, then read any surrounding tests or diagnostics. Update the user-facing panic guidance to recommend time_span::Span, and verify that the message clearly points users toward the intended replacement.