Finish up branch implementation

* while and unconditional loops are now supported fully
* break and continue keywords for loop control
* List::thunkify() has been broken into its own structure so it can be
  broken out further as necessary

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

src/vm/inst.rs

@@ -42,10 +42,11 @@ pub enum Inst {
     Jump(usize),
 
     /// Jump to a given address in the current function if the condition flag is true.
-    ///
-    /// The condition flag may be set by an internal function.
     JumpTrue(usize),
 
+    /// Jump to a given address in the current function if the condition flag is false.
+    JumpFalse(usize),
+
     /// Calls a function with the supplied number of arguments.
     ///
     /// The stack, from bottom to top, should contain the function followed by the arguments.
@@ -83,6 +84,7 @@ impl Inst {
             Inst::CheckTruth => "CHECK_TRUTH",
             Inst::Jump(_) => "JUMP",
             Inst::JumpTrue(_) => "JUMP_TRUE",
+            Inst::JumpFalse(_) => "JUMP_FALSE",
             Inst::Call(_) => "CALL",
             Inst::Index => "INDEX",
             Inst::Return => "RETURN",