Make IOEnv monad one-shot (#18202)
On CI (x86_64-linux-deb9-hadrian, compile_time/bytes_allocated):
T10421 -1.8% (threshold: +/- 1%)
T10421a -1.7% (threshold: +/- 1%)
T12150 -4.9% (threshold: +/- 2%)
T12227 -1.6 (threshold: +/- 1%)
T12425 -1.5% (threshold: +/- 1%)
T12545 -3.8% (threshold: +/- 1%)
T12707 -3.0% (threshold: +/- 1%)
T13035 -3.0% (threshold: +/- 1%)
T14683 -10.3% (threshold: +/- 2%)
T3064 -6.9% (threshold: +/- 2%)
T4801 -4.3% (threshold: +/- 2%)
T5030 -2.6% (threshold: +/- 2%)
T5321FD -3.6% (threshold: +/- 2%)
T5321Fun -4.6% (threshold: +/- 2%)
T5631 -19.7% (threshold: +/- 2%)
T5642 -13.0% (threshold: +/- 2%)
T783 -2.7 (threshold: +/- 2%)
T9020 -11.1 (threshold: +/- 2%)
T9961 -3.4% (threshold: +/- 2%)
T1969 (compile_time/bytes_allocated) -2.2% (threshold: +/-1%)
T1969 (compile_time/max_bytes_used) +24.4% (threshold: +/-20%)
Additionally on other CIs:
haddock.Cabal -10.0% (threshold: +/- 5%)
haddock.compiler -9.5% (threshold: +/- 5%)
haddock.base (max bytes used) +24.6% (threshold: +/- 15%)
T10370 (max bytes used, i386) +18.4% (threshold: +/- 15%)