GPU backend

Adds GPU backend and "CUDA" feature to all the tests. As of MR creation, there were no forks in tested IRs so only vanilla non-parallel (single block, single thread) codegen has been tested.

hercules_cg/src/gpu.rs

@@ -8,9 +8,6 @@ use self::hercules_ir::*;
 
 use crate::*;
 
-use std::fs::OpenOptions;
-use std::io::Write as IoWrite;
-
 /*
  * The top level function to compile a Hercules IR function into CUDA
  * kernel for execution on the GPU. We generate CUDA C textually, with a lot