Reduce peak memory usage

[Fred Emmott]

Some possibilities:

• add an option to hh_parse to not include program_text in the json output
  • do we need $json_data['program_text'] at all?
• migrate to using s-expressions instead of json
• modify File so only load content on-demand (unless the file is dirty)

[Fred Emmott]

@KendallHopkins any thoughts/internal experiments on these?