General changes across the runtime crate in support of compile module

* Compile module is able to compile bytecode (or so it seems...)
* Runtime crate has had some new stuff added to it, mostly with objects
  and vtables. Still not 100% on the object method function call story,
  but I guess it'll be tackled when we get there.

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

src/compile/error.rs

@@ -0,0 +1,12 @@
+use snafu::Snafu;
+
+#[derive(Debug, Snafu)]
+pub enum Error {
+    #[snafu(display("invalid assignment target"))]
+    InvalidLhs,
+
+    #[snafu(display("invalid object attribute"))]
+    InvalidAttr,
+}
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;