Improving the combine mode for hpc

Improving the combine mode for hpc

we now have
Processing Coverage files:
  sum         Sum multiple .tix files in a single .tix file
  combine     Combine two .tix files in a single .tix file
  map         Map a function over a single .tix file

Where sum joins many .tix files, combine joins two files (with
extra functionality possible), and map just applied a function
to single .tix file.

These changes were improvements driven by hpc use cases.

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This patch contains the following changes:

M ./utils/hpc/Hpc.hs -1 +3
M ./utils/hpc/HpcCombine.hs -33 +84
M ./utils/hpc/HpcFlags.hs -11 +59