- Attach a SrcSpan to every CostCentre.  This had the side effect
    that CostCentres that used to be merged because they had the same
    name are now considered distinct; so I had to add a Unique to
    CostCentre to give them distinct object-code symbols.

  - New flag: -fprof-auto-calls.  This flag adds an automatic SCC to
    every call site (application, to be precise).  This is typically
    more useful for call stacks than annotating whole functions.

Various tidy-ups at the same time: removed unused NoCostCentre
constructor, and refactored a bit in Coverage.lhs.

The call stack we get from traceStack now looks like this:

Stack trace:
  Main.CAF (<entire-module>)
  Main.main.xs (callstack002.hs:18:12-24)
  Main.map (callstack002.hs:13:12-16)
  Main.map.go (callstack002.hs:15:21-34)
  Main.map.go (callstack002.hs:15:21-23)
  Main.f (callstack002.hs:10:7-43)