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/frame.rs

@@ -1,6 +1,52 @@
+use crate::obj::prelude::*;
+use shredder::{GcSafe, Scan, Scanner};
+use std::collections::BTreeMap;
+
 /// A stack call frame.
-#[derive(Default, Debug, Clone)]
-pub struct Frame {
-    last_pc: usize,
-    stack_base: usize,
+#[derive(Debug, Clone)]
+pub enum FrameKind {
+    Root,
+    Native,
+    User {
+        last_pc: usize,
+        stack_base: usize,
+        fun: UserFunRef,
+    },
+}
+
+unsafe impl Scan for FrameKind {
+    fn scan(&self, scanner: &mut Scanner<'_>) {
+        match self {
+            FrameKind::User { fun, .. } => scanner.scan(fun),
+            _ => { /* no-op */ }
+        }
+    }
+}
+
+unsafe impl GcSafe for FrameKind {}
+
+pub type Locals = BTreeMap<usize, ObjRef>;
+
+#[derive(Scan, Debug, Clone)]
+pub struct Frame {
+    locals: Locals,
+    kind: FrameKind,
+}
+
+impl Frame {
+    pub fn new(locals: Locals, kind: FrameKind) -> Self {
+        Self { locals, kind, }
+    }
+
+    pub fn locals(&self) -> &Locals {
+        &self.locals
+    }
+
+    pub fn locals_mut(&mut self) -> &mut Locals {
+        &mut self.locals
+    }
+    
+    pub fn kind(&self) -> &FrameKind {
+        &self.kind
+    }
 }