Make a new body.h file with some functions to deal with pod and their
body. Make the iter.h functions use mostly this.
Rework the parser so that it only uses body.h functions. With the separation
of pod+body, we can read and verify the pod once and then use the
verified copy to handle the rest of the body safely.
We do this because iter.h only works in pods in memory that doesn't change
because it is vulnerable to modifications of the data after verifying it.
The new parser is not vulnerable to this and will not cause invalid
memory access when used on shared memory. There is however no need for
atomic operations to read the headers, whever is read is either valid
and useable of invalid and rejected.
See #4822
