Add some base VM implementations

Some instructions are currently implemented. Others are not. This is
mostly just a checkpoint so I can implement lexical name definitions.

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

src/vm/error.rs

@@ -0,0 +1,9 @@
+use snafu::Snafu;
+
+#[derive(Debug, Snafu)]
+pub enum Error {
+    #[snafu(display("attempted to pop an empty stack"))]
+    EmptyStack,
+}
+
+pub type Result<T, E = Error> = std::result::Result<T, E>;