Split Trace.Hpc.Combine into three modules

• Use hierarchical modules Trace.Hpc.Foo instead of flat HpcFoo.
• Add Haddock documentation headers to modules
• Move the Plugin type from Trace.Hpc.Flags to its own module Trace.Hpc.Plugin
• Split Trace.Hpc.Combine into three modules for the three subcommands map, sum and combine.