[fidl][difl] Moving towards Tricium support

New features:
 - Generate Tricium proto compatible JSON output.
 - Consume a list of paths to JSON IR that's produced by the build system.

Given a couple of built Fuchsia trees:
  ./scripts/difl --before-files=out/fidl-json-bak/all_fidl_json.txt \
                 --after-files=out/x64/all_fidl_json.txt \
                 --format=tricium
will describe the ABI changes between the two in a Tricium-friendly JSON
format.

Given an unbuilt tree that has run `gn gen`:
  ninja -C out/x64 $(cat out/x64/all_fidl_json.txt)
will just build the FIDL IR. Note: It'll expect to already have `fidlc`
from a Zircon build.

Change-Id: I4d56706ba4740bd24df76f7bee5de41d0e8fa306