Use GHC 9.0's `QualifiedDo` extension instead of `RebindableSyntax`

[alex-lew]

The trace types paper uses the syntax M.do and P.do to distinguish the monad of measures from the graded monad of traced probabilistic programs. With the upcoming release of GHC 9, it will be possible to directly support this syntax in the implementation, thanks to the QualifiedDo extension.