Switch parachain interface to new runtime_interface macro (#665)

* Make use of `runtime_interface` for parachain externalities

This also changes the encoding of the `ValidationResult` return value to
match the default encoding used in Substrate.

* Fix compilation for web

* Update `Cargo.lock`

* Include feedback

* Move proc macro

* Update parachain/src/lib.rs

Co-Authored-By: Robert Habermeier <rphmeier@gmail.com>

Co-authored-by: Robert Habermeier <rphmeier@gmail.com>

polkadot/test-parachains/halt/build.rs

@@ -20,6 +20,6 @@ fn main() {
 	build_current_project_with_rustflags(
 		"wasm_binary.rs",
 		WasmBuilderSource::Crates("1.0.7"),
-		"-C link-arg=--import-memory",
+		"-C link-arg=--export=__heap_base",
 	);
 }