assignment: Add variable assignment

* Syntax for x = y
* Compiler creates Inst::Store thunks
* New Vm::store function

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

src/syn/ast.rs

@@ -2,7 +2,7 @@ use crate::syn::Spanned;
 
 #[derive(Debug, Clone)]
 pub enum Stmt {
-    Assign(AssignLhs, Expr),
+    Assign(SpAssignLhs, SpExpr),
     Expr(SpExpr),
     If {
         if_true: SpCondExpr,
@@ -20,6 +20,8 @@ pub enum Stmt {
     },
 }
 
+pub type SpAssignLhs = Spanned<AssignLhs>;
+
 #[derive(Debug, Clone)]
 pub enum AssignLhs {
     /// A simple assignment.