misc/substring.h: Refactor with MemRef.
ncd: modules: explode: Optimize by not copying the input string.
ncd: modules: explode: Pass through external strings (avoid copy).
CMakeLists.txt: Don't build the -plugin libs by default.
ncd: modules: value: Expose strings as referencable external-strings.
This means we can use e.g. explode() in them efficiently.
ncd: modules: value: Refactoring with MemRef.
ncd: NCDStringIndex: Refactor using MemRef.