Infer function expressions in matching contexts (#514)

* legalizes omitting types on function expressions within function type contexts
* legalizes omitting any number of arguments

tests/compiler/function-expression.ts

@@ -14,3 +14,23 @@ f3();
 
 var f4 = (): i32 => 1;
 assert(f4() == 1);
+
+function testOmitted(fn: (a: i32, b: i32) => i32): i32 {
+  return fn(1, 2);
+}
+assert(testOmitted((a, b) => a + b) == 3);
+assert(testOmitted(a => a) == 1);
+assert(testOmitted(() => 42) == 42);
+
+function testOmittedReturn1(): (a: i32, b: i32) => i32 {
+  return (a, b) => a + b;
+}
+function testOmittedReturn2(): (a: i32, b: i32) => i32 {
+  return a => a;
+}
+function testOmittedReturn3(): (a: i32, b: i32) => i32 {
+  return () => 42;
+}
+assert(testOmittedReturn1()(1, 2) == 3);
+assert(testOmittedReturn2()(1, 2) == 1);
+assert(testOmittedReturn3()(1, 2) == 42);