Each header should only include the other headers it needs to compile;
`devtools/reduce-includes.sh */*.h` does this. The C files then need
additional includes if they don't compile.
And remove the entirely useless wire/onion_wire.h, which only serves to include wire/onion_wiregen.h.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
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