A Writer represents a consumer of the bytes produced by serializing a message. In cases where you can
create a slice to put the bytes in, CursorWriter can be used. Custom Writers are useful for more niche
uses, such as sending the serialized bytes over a serial port.
Deserialize a T from some bytes and, optionally, some handles. If the wire is not able to transport handles,
it is fine to produce &[] (as long as T does not contain any handles, that is).
It can sometimes be useful to know the size of a value in its serialized form (e.g. to reserve space for it in
a ring buffer). This calculates the number of bytes taken to serialize some value of T into Ptah’s wire
format. Note that this size is for the specific value, and may differ between values of T.