[fine] Check the return type of functions

A function returns what it says it does, the check is that the body
returns the right value.

fine/tests/expression/arithmetic.fine

@@ -1,4 +1,4 @@
-fun test() {
+fun test() -> f64 {
     1 * 2 + -3 * 4
 }
 
@@ -13,6 +13,10 @@ fun test() {
 // |     ParamList
 // |       LeftParen:'"("'
 // |       RightParen:'")"'
+// |     ReturnType
+// |       Arrow:'"->"'
+// |       TypeExpression
+// |         Identifier:'"f64"'
 // |     Block
 // |       LeftBrace:'"{"'
 // |       ExpressionStatement