Increase precision of timings reported by RTS

Summary:
Today's hardware is much faster, so it makes sense to report timings
with more precision, and possibly help reduce rounding-induced
fluctuations in the nofib statistics.

This commit increases the precision of all timings previously reported
with a granularity of 10ms to 1ms. For instance, the `+RTS -S` output is
now rendered as:

    Alloc    Copied     Live     GC     GC      TOT      TOT  Page Flts
    bytes     bytes     bytes   user   elap     user     elap
   641936     59944    158120  0.000  0.000    0.013    0.001    0    0  (Gen:  0)
   517672     60840    158464  0.000  0.000    0.013    0.002    0    0  (Gen:  0)
   517256     58800    156424  0.005  0.005    0.019    0.007    0    0  (Gen:  1)
   670208      9520    158728  0.000  0.000    0.019    0.008    0    0  (Gen:  0)

  ...

                                     Tot time (elapsed)  Avg pause  Max pause
  Gen  0        24 colls,     0 par    0.002s   0.002s     0.0001s    0.0002s
  Gen  1         3 colls,     0 par    0.011s   0.011s     0.0038s    0.0055s

  TASKS: 4 (1 bound, 3 peak workers (3 total), using -N1)

  SPARKS: 0 (0 converted, 0 overflowed, 0 dud, 0 GC'd, 0 fizzled)

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    0.005s  (  0.006s elapsed)
  GC      time    0.014s  (  0.014s elapsed)
  EXIT    time    0.001s  (  0.001s elapsed)
  Total   time    0.032s  (  0.020s elapsed)

Note that this change also requires associated changes in the nofib
submodule.

Test Plan: tested with modified nofib

Reviewers: simonmar, nomeata, austin

Subscribers: simonmar, relrod, carter

Differential Revision: https://phabricator.haskell.org/D97