Avoid boxing struct during serialization

[steveharter]

As part of #2259, we deferred fixing an issue that causes an extra box+unbox when a struct is serialized. This adds ~32-40 bytes of allocation plus slows the execution.

Expect:

• No boxing occurs for simple value structs (a single-value struct with no properties; just a custom value type). Note boxing occurs here in 3.0\3.1 as well (a single box+unbox), so we will improve on that in 5.0 by removing it.
• A single box+unbox for "POCO structs" where the struct has properties that are serialized. Currently we have 2. This is a regression from 3.0\3.1 where we only had 1.