Utility functions for marshalling to and from bigstring.
In all functions below, pos is the index into the bigstring to read from or write to
and an exception is raised if that index is invalid. The default is 0.
marshals value _ to the bigstring at most len bytes.
marshals value _ to a new bigstring. This function may need two times more memory
the length of marshalled data in the bigstring
unmarshals a value from the bigstring and/or returns the index of the byte in the
bigstring right after the unmarshalled value.