Fold runtime/ crate into this source tree

While I like the idea of having a runtime completely decoupled from the
syntax and compiler, I don't think this is that big of a project for
that to be necessary or even useful yet.

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

src/vm/frame.rs

@@ -0,0 +1,6 @@
+/// A stack call frame.
+#[derive(Default, Debug, Clone)]
+pub struct Frame {
+    last_pc: usize,
+    stack_base: usize,
+}