The cstring datatype can now be copied, passed around, etc. The typlen

The cstring datatype can now be copied, passed around, etc.  The typlen
value '-2' is used to indicate a variable-width type whose width is
computed as strlen(datum)+1.  Everything that looks at typlen is updated
except for array support, which Joe Conway is working on; at the moment
it wouldn't work to try to create an array of cstring.