Implement optional type parameters (#360)

* Add a NATIVE<T> macro type to simplify use of a native WebAssembly type
* Add default type parameters for internal helpers for explicit loads and stores
* Unify loadUnsafe/loadUnsafeWithOffset etc. into one
* Renamed loadUnsafe etc. into just LOAD, like a macro
* Implement parsing of index signatures, but ignore them, for properly linting code
* Refactor TypedArray<T> to use macros

tests/compiler/optional-typeparameters.ts

@@ -0,0 +1,28 @@
+function testConcrete<T,U = i32>(a: T): U {
+  return a;
+}
+
+function testDerived<T,U = T>(a: T): U {
+  return a;
+}
+
+testConcrete<i32>(1);
+testDerived<i32>(2);
+
+class TestConcrete<T,U = i32> {
+  test<V = i32>(a: T, b: U): V {
+    return a + b;
+  }
+}
+
+class TestDerived<T,U = T> {
+  test<V = U>(a: T, b: U): V {
+    return a + b;
+  }
+}
+
+import "allocator/arena";
+var tConcrete = new TestConcrete<i32>();
+tConcrete.test<i32>(1, 2);
+var tDerived = new TestDerived<f64>()
+tDerived.test<f64>(1, 2);