Go 1.20 switched to uint64 to represent goroutine IDs, we can't
actually follow suit because we have allowed clients to use -1 to refer
to the currently selected goroutine, however we should at least switch
to int64 and also update the rtype check to accept the 1.20 type.
Adds a script that check automatically that the the assumptions that
pkg/proc makes about runtime are met by the actual runtime, using a
combination of parsing and magic comments.
Also emits a file describing all the struct fields, constants and
variables of the runtime that we use in pkg/proc.
