Return a const char * from read_file_mapping::read
Use read_file_mapping for the thesaurus
Replace all uses of strstream and stringstream with bufferstream

istringstream makes a copy of its input, which is not very nice for
performance. strstream doesn't do bounds checking, which is not very
nice for safety (and is deprecated).
Don't wrap the default config json blobs in std::strings
Kill default_path.json since it was never used for anything
Expunge remaining references to <fstream> and <iostream>
Remove old cruft from the precompiled headers