This fixes the problem with duplicate cost-centre names that was
reported a couple of times before. When a module implements a typeclass
multiple times for different types, methods of different implementations
get same cost-centre names and are reported like this:

    COST CENTRE MODULE            %time %alloc

    CAF         GHC.IO.Handle.FD    0.0   32.8
    CAF         GHC.Read            0.0    1.0
    CAF         GHC.IO.Encoding     0.0    1.8
    showsPrec   Main                0.0    1.2
    readPrec    Main                0.0   19.4
    readPrec    Main                0.0   20.5
    main        Main                0.0   20.2

                                            individual      inherited
    COST CENTRE  MODULE  no.     entries  %time %alloc   %time %alloc

    MAIN         MAIN     53          0    0.0    0.2     0.0  100.0
     CAF         Main    105          0    0.0    0.3     0.0   62.5
      readPrec   Main    109          1    0.0    0.6     0...