Document more TODOs as tickets (#1418)

Went through the TODOs, removed a bunch, which are outdated or nothing more than a regular comment, documented a bunch more as actual tickets and made them FIXMEs and unified their structure (`FIXME #TICKETNO DESC` for local tickets, `FIXME: DESC LINK` for external tickets) for easier in-editor support. Further more remove unnecessary remarks and related old code that I noticed in that instance.

substrate/core/executor/src/native_executor.rs

@@ -268,7 +268,6 @@ macro_rules! native_executor_instance {
 		native_executor_instance!(IMPL $name, $dispatcher, $version, $code);
 	};
 	(IMPL $name:ident, $dispatcher:path, $version:path, $code:expr) => {
-		// TODO: this is not so great – I think I should go back to have dispatch take a type param and modify this macro to accept a type param and then pass it in from the test-client instead
 		use primitives::Blake2Hasher as _Blake2Hasher;
 		impl $crate::NativeExecutionDispatch for $name {
 			fn native_equivalent() -> &'static [u8] {

substrate/core/executor/src/wasm_executor.rs

@@ -118,7 +118,6 @@ impl ReadPrimitive<u32> for MemoryInstance {
 	}
 }
 
-// TODO: this macro does not support `where` clauses and that seems somewhat tricky to add
 impl_function_executor!(this: FunctionExecutor<'e, E>,
 	ext_print_utf8(utf8_data: *const u8, utf8_len: u32) => {
 		if let Ok(utf8) = this.memory.get(utf8_data, utf8_len as usize) {