Merge ClientIp with Headers.

Since we now use the `ClientIp` Guard on a lot more places, it also
increases the size of binary, and the macro generated code because of
this extra Guard. By merging the `ClientIp` Guard with the several
`Header` guards we have it reduces the amount of code generated
(including LLVM IR), but also a small speedup in build time.

I also spotted some small `json!()` optimizations which also reduced the
amount of code generated.