Allocate all memories up-front

• Refactor memory allocation strategy for generated Hercules code
  • Allocate device memory up-front on calling a Hercules function
• Introduce HerculesRef types and HerculesRunner generated types for safe Rust interface
• Large refactor of GCM, includes base code for object allocation
• Enhancements for graphviz visualization

hercules_cg/src/device.rs

-use hercules_ir::*;
-
-/*
- * Top level function to definitively place functions onto devices. A function
- * may store a device placement, but only optionally - this function assigns
- * devices to the rest of the functions.
- */
-pub fn device_placement(functions: &Vec<Function>, callgraph: &CallGraph) -> Vec<Device> {
-    let mut devices = vec![];
-
-    for (idx, function) in functions.into_iter().enumerate() {
-        if let Some(device) = function.device {
-            devices.push(device);
-        } else if function.entry || callgraph.num_callees(FunctionID::new(idx)) != 0 {
-            devices.push(Device::AsyncRust);
-        } else {
-            devices.push(Device::LLVM);
-        }
-    }
-
-    devices
-}