Compiling and running the Main module

GHC allows you to determine which module contains the "main" function, and what that function is called, via the -fmain-is flag. The trouble is that the runtime system is fixed, so what symbol should it link to?

The current solution is this. Suppose the main function is Foo.run.

Then, when compiling module Foo, GHC adds an extra definition:
```
  :Main.main = runIO Foo.run
```
Now the RTS can invoke :Main.main to start the program. (This extra definition is inserted in TcRnDriver.checkMain.)
Before starting the program, though, the RTS also initialises the module tree by calling init_:Main, so when compiling the main module (Foo in this case), as well as generating init_Foo as usual, GHC also generates
```
  init_zcMain() { init_Foo; }
```
This extra initialisation code is generated in CodeGen.mkModuleInit.