Make feldspar-compiler a plugins-multistage client

With plugins-multistage it is now possible to use the `loadFun` from
`Feldspar.Compiler.Plugin` to dynamically compile and load Feldspar
functions into Haskell sessions.
Use explicit ghc
Lookup runtime header files in package databases or locally
Document the Marshal module
Remove debug printout
Refine comment
Add Marshal and Reference instances for tuples
Add CRC benchmark

Benchmark the CRC implementation
 * Using the Feldspar evaluator
 * Using compiled code by loading it using plugins.
Add Default instance for Ptr and SA
Whitespace
Initialize (zero) memory allocated by initArray
Allow null pointers in freeArray
Fix plugin interpretation of the calling convention
Add Calling Convention test suite
Bump plugins-multistage lower bound
Use `with def` instead of `alloca`
Merge branch 'master' into plugins2

Conflicts:
	lib/Feldspar/C/feldspar_array.h
Update tests and benchmarks to multidim vectors
Remove use of deprecated Vector1
Relax lower bound on directory