Add a new `json_parser_load_from_mapped_file()` to load JSON from
files via memory mapping. It’s otherwise similar to
`json_parser_load_from_file()`. It’s in the right position to be able
to memory map the file it’s reading from: it reads the input once
before building a `JsonNode` structure to represent it, doesn’t write
to the file, and often deals with large input files.

This should speed things up slightly due to reducing time spent
allocating a large chunk of heap memory to load the file into, if a
caller can support that.

Signed-off-by: Philip Withnall <withnall@endlessm.com>