Build and install executable section

[phischu]

When attempting to install an executable with haskell-suite cabal just dies. I believe the following steps are necessary to allow custom compilers to also process executable sections:

• Design a command line interface for executable building.
• Design a command line interface for executable installation.
• In Cabal write a function 'Distribution.Simple.HaskellSuite.buildExe' and a function 'Distribution.Simple.HaskellSuite.installExe'. They should call the compiler tool.
• Invent reasonable defaults for the 'simple' compiler.
  Perhaps in the future I might propose something concrete in the form of a pull request.