[DDMD] Rename StringTable::init -> _init

Using a method called `init` in D code causes problems, because `init` is a built-in property in D.  In most cases it can be automatically converted, but `StringTable::init` is used in the glue layer, so the name mangling in D and C++ must match exactly.  The simplest answer is to rename it in the C++ code, 8 lines changed.