[fine] Assignments!

And new error capabilities!

fine/tests/expression/assignment.fine

@@ -0,0 +1,34 @@
+fun test() -> f64 {
+    let x = 12;
+    let y = 13;
+    let z = 2;
+    x = y = z;
+    x
+}
+
+// @no-errors
+// @compiles-to:
+// | function << module >> (0 args, 0 locals):
+// |   strings (0):
+// |   code (2):
+// |     0: PushNothing
+// |     1: Return
+// | function test (0 args, 3 locals):
+// |   strings (0):
+// |   code (14):
+// |     0: PushFloat(12.0)
+// |     1: StoreLocal(0)
+// |     2: PushFloat(13.0)
+// |     3: StoreLocal(1)
+// |     4: PushFloat(2.0)
+// |     5: StoreLocal(2)
+// |     6: LoadLocal(2)
+// |     7: Dup
+// |     8: StoreLocal(1)
+// |     9: Dup
+// |     10: StoreLocal(0)
+// |     11: Discard
+// |     12: LoadLocal(0)
+// |     13: Return
+// |
+// @eval: Float(2.0)