Adds Mavis support to read trace in JSON form (#14)

* Mavis added as a submodule

* Mavis is installed and compiling!  Not used yet

* Mavis instantiated; complains about no uarch files: good!

* Updated mavis to gcc branch fixes

* Mavis is now installed and loading arch files

* Workload added and passed to Fetch

* Now have json files running

* Olympia can now run json traces

* Updated documentation -- still more work to do

* Updated documentation -- still more work to do

* Updated documentation -- still more work to do

* Declare InstPtr type within olympia namespace

* Removed outdated fetch param; tests still failing

* Added json test in regress

* Fixed LLVM compilation warnings

* Fixed conflicts

Co-authored-by: Knute Lingaard <Knute.Lingaard@sifive.com>
Co-authored-by: zen <zhen.wei@sifive.com>