json-parser: Support loading files via memory mapping
json_parser_load_from_file() is in the right position to be able to
memory map the file it’s loading from: it reads the input once before
JsonNode structure to represent it, doesn’t write to the
file, and often deals with large input files.
Try memory mapping a file before falling back to loading it manually. This should speed things up slightly due to reducing time spent allocating a large chunk of heap memory to load the file into.
Signed-off-by: Philip Withnall email@example.com