remove DOES's usage of SvSCREAM

remove DOES's usage of SvSCREAM

Currently the SvSCREAM flag is set on a temporary SV whose string value
is "isa", but where for the purposes of printing

    Can't call method "XXX"

its name is treated as "DOES" rather than "isa".

Instead, set the temp SV's PVX buffer to point to a special static
string (PL_isa_DOES) whose value is "isa", but the where the error
reporting code can compare the address with PL_isa_DOES and if so, print
"DOES" instead.

This is to reduce the number of odd special cases for the SvSCREAM flag.