handle unsized by-ref values

[0xmuon]

added handling for unsized by-ref values (CValueInner::ByRef(_, Some(meta))) in CValue::value_field and removes the remaining todo!() crash paths in src/value_and_place.rs. For write_cvalue from an unsized by-ref into a sized destination, it falls back to copying the sized prefix (destination’s fixed size) to avoid hard failures.

[0xmuon]

Is “copy sized prefix when writing from ByRef(_, Some(meta)) into a sized place” fallback acceptable or would you prefer we span_bug! there instead to enforce that such writes never happen?

[bjorn3]

Please update the PR title and description to match the contents of the PR. Other than that LGTM.

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