Add object layout, object parsing, instruction layout

* Object layout and parsing are done in the vm::obj module
* Add MemCursor, a wrapper around the std::io::Cursor type for walking
  through VM memory
* Add vm::tick module for containing the Vm::tick() method
  implementation, since it's pretty big
* Instructions are now variable-sized, and are read lazily,
  one-at-a-time directly from memory.
* Add VM runtime error structure
* Probably some other stuff I forgot

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/vm/error.rs

@@ -0,0 +1,9 @@
+use snafu::Snafu;
+use crate::vm::vm::*;
+
+#[derive(Snafu, Debug, Clone)]
+pub enum VmError {
+    MemOutOfBounds { addr: Addr, }
+}
+
+pub type Result<T, E = VmError> = std::result::Result<T, E>;