[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/conditional.fine

@@ -1,22 +1,22 @@
-fun test() {
+fun test() -> f64 {
     if true { "discarded"; 23 } else { 45 }
 }
 
 // @no-errors
 // Here come some type probes!
 // (type of the condition)
-// @type: 20 bool
+// @type: 27 bool
 //
 // (the discarded expression)
-// @type: 27 string
+// @type: 34 string
 //
 // (the "then" clause)
-// @type: 40 f64
-// @type: 43 f64
+// @type: 47 f64
+// @type: 50 f64
 //
 // (the "else" clause)
-// @type: 52 f64
-// @type: 55 f64
+// @type: 59 f64
+// @type: 62 f64
 //
 // @concrete:
 // | File
@@ -26,6 +26,10 @@ fun test() {
 // |     ParamList
 // |       LeftParen:'"("'
 // |       RightParen:'")"'
+// |     ReturnType
+// |       Arrow:'"->"'
+// |       TypeExpression
+// |         Identifier:'"f64"'
 // |     Block
 // |       LeftBrace:'"{"'
 // |       IfStatement