Bug fixes in shader group parsing:
1. failed to consider layer names containing characters not found in C
identifiers.
2. Botched unescaping of quoted strings.
Allow queries of a group's name, and number/names of its layers
Switch OSLCompilerImpl::oso() to tinyformat template, now it's type safe
Allow OSLQuery from inside ShadingSystem
Asking for OSLQuery to be filled out for a loaded shader (specified by
ShaderGroup* and layer index) is much faster than using liboslquery
to read oso from disk.
Overhaul OSLQuery to replace std::string with ustring and string_view.
This should lead to fewer small allocations and more efficient setup
of OSLQuery.