`AsBytes::as_bytes` not amenable to inlining

[antiguru]

Rust doesn't like inlining the iterator returned from AsBytes::as_bytes, likely because it's too deeply nested with a large number of Chains. This affects length_in_bytes equivalently. We should look into how to improve the situation, for example by accepting a closure instead of returning an iterator.

[frankmcsherry]

Is there an easy repro, without end-to-end MZ work? Mostly thinking that some forced inlining could work, which accepting a closure would do I think (because of code gen), but also it seems like there shouldn't be function calls here on account of the iterator is just unpacking the fields in the structure, rather than anything particularly complicated.