Initial heavy hitters support via Mastic

[cjpatton]

~Based on #574 (merge that first).~

This is a prototype of proposal 1 from: https://docs.google.com/document/d/1ZjXz-1kGsTDf2Vn2u-fwYqR8BSc3tOYlELVHAYvAfjk

Daphne will use the Mastic VDAF rather than Poplar1 because we only support 1-round VDAFs. (Poplar1 requires two rounds for preparation.) For now we're working with a dummy version of Mastic, as its implementation is still in progress: https://github.com/divviup/libprio-rs/issues/947

This is not ready to merge. We need to:

• [ ] Align with the changes we settle on in the DAP specification (some changes will be required if we take proposal 2)
• [ ] Consider fencing changes to protocol and messages behind the "test-utils" feature
• [ ] Plumb the new code path through roles module
• [ ] Additional tests
  • Leader tries to override an existing report
  • Stored report does not exist
  • ...
• [ ] Replace dummy Mastic with the real one
• [ ] Implement VIDPF caching