Perf: make SDoc monad one-shot (#18202)
With validate-x86_64-linux-deb9-hadrian:
T1969 -3.4% (threshold: +/-1%)
T3294 -3.3% (threshold: +/-1%)
T12707 -1.4% (threshold: +/-1%)
Additionally with validate-x86_64-linux-deb9-unreg-hadrian:
T4801 -2.4% (threshold: +/-2%)
T13035 -1.4% (threshold: +/-1%)
T13379 -2.4% (threshold: +/-2%)
ManyAlternatives -2.5% (threshold: +/-2%)
ManyConstructors -3.0% (threshold: +/-2%)