While refactoring the IPC encoders and decoders for fallibility, the
inconsistency in which we transfer container sizes was a frequent thing
to trip over. We currently transfer sizes as any of i32, u32, and u64.
This adds a helper to transfer sizes in one consistent way.
Two special cases here are DeprecatedString and Vector, whose encoding
is depended upon by netdb, so that is also updated here.
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