[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)

fine/tests/expression/errors/assignment_errors.fine

@@ -0,0 +1,20 @@
+fun something() { }
+fun something_else() { }
+
+fun wrong() {
+  let x = 12;
+  let y = "hello";
+  x = y; // should be an error
+  y = x; // should be an error
+
+  let z = 23;
+  y = x = z; // should be an error
+
+  something_else = something; // should be an error
+}
+
+// @expect-errors:
+// | 7:4: cannot assign a value of type `string` to type `f64`
+// | 8:4: cannot assign a value of type `f64` to type `string`
+// | 11:4: cannot assign a value of type `f64` to type `string`
+// | 13:2: cannot assign a new value to a function declaration