Adding the new chunked file format (CFF) system.

This is the initial work on the new file format for WTF.

It adds a new wtf.io.cff class tree that implements the format as described in the [docs/wtf-trace.md](https://github.com/google/tracing-framework/blob/cff/docs/wtf-trace.md) doc. It supports both reading and writing JSON and binary formats.

This new format is working enough to save traces and load them, however there is a lot of plumbing work required to get it working everywhere (node, cross-extension, popup, etc). This work requires API changes and a lot of code deduplication and will be done in future changes on this branch. The core of the file format will not be changing, though, so the intent is to get this reviewed and move forward before merging to master.