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/parser/index-declaration.ts.fixture.ts

@@ -0,0 +1,3 @@
+class A {
+  [key: i32]: f64;
+}