Fix#77
* Check MSRV in CI
* Lower MSRV to 1.71.1 by gating `allow(..., reason = "...")`
* Sets all min dependency crate versions needed by riven, via `-Z
direct-minimal-versions`
* Adds `test-min-deps.bash` script to run `-Z direct-minimal-versions`
* Removes underutilized `scan_fmt` dependency, replaces with simple
string manipulation code
