Implement Implicit prelude (#224)

Summary of changes:

* Add a desugar step to implicitly import the Prelude
* Remove the `-XNoImplicitPrelude` flag from GHC's command line when typechecking
* Add a test to verify that the Prelude is imported implicitly
* Add an argument to `compileWith` to allow desugaring before processing imports
* Fix any other tests which were broken by these things

Todo:

* There are probably some redundant calls to `desugar` (since I added one in `Fay.Compiler.Import:compileWith`)
* Refactoring?