I'm thinking that a CPS-style parser type could allow returning an
unboxed value as a result of the compiler inlining and fusing together
the parsing code and the code that consumes the parsed value.
Are there any libraries that work like this?
Both cereal (for binary) and attoparsec (text) are written in a CPS style that can support this in principle. For parsers of even modest complexity, GHC won't necessarily succeed at unboxing values, though; you have to inspect the Core to see if what's going on matches what you're hoping for.