Decimal represents a 128 bit representation of a fixed-precision decimal number.
The finite set of values of type Decimal are of the form m / 10e,
where m is an integer such that -296 < m < 296, and e is an integer
between 0 and 28 inclusive.
One ServerMessage is supposed to hold as much data as possible
without delaying updates, but still avoiding going too far above
than 1M / message, to allow for better resumability.
Note that for this trait to be implemented correctly, the Eq trait should be
equivalent to equality of the JSON-serialized representation. In particular, this
means that things like [HashMap]s should be banned, and [BTreeMap]s should be
preferred.