More string_ref use for ParamValue
Use string_ref for ImageSpec::attribute/getattribute and global OIIO::attribute/getattribute
string_ref-ify ImageSpec::attribute(name,type,stringvalue)
As it turned out, this needed an overhaul of the implementation and its
test, it was quite wrong in how it parsed non-string values from string.
Use string_ref when passing attributes for ImageCache and TextureSystem
Minor ustring/string_ref tweaks
Improve string_ref and ustring documentation
string_ref: make substr past end just return empty instead of exception