[GHC] #8861: Use commas to separate thousands when printing memory stats

[GHC]

#8861: Use commas to separate thousands when printing memory stats
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        Reporter:  ErlendH     |            Owner:
            Type:  feature     |           Status:  patch
  request                      |        Milestone:
        Priority:  lowest      |          Version:  7.6.3
       Component:  GHCi        |         Keywords:
      Resolution:              |     Architecture:  Unknown/Multiple
Operating System:              |       Difficulty:  Easy (less than 1 hour)
  Unknown/Multiple             |       Blocked By:
 Type of failure:              |  Related Tickets:
  None/Unknown                 |
       Test Case:              |
        Blocking:              |
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Comment (by rwbarton):

 `./prog +RTS -s` already prints the memory stats with commas, can you
 reuse whatever code does the formatting there?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8861#comment:3>
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