wasm-builder: Make it easier to build a WASM binary (#4177)

Basically combines all the recommended calls into one
`build_using_defaults()` call or `init_with_defaults()` when there are
some custom changes required.

polkadot/runtime/test-runtime/build.rs

@@ -17,9 +17,5 @@
 use substrate_wasm_builder::WasmBuilder;
 
 fn main() {
-	WasmBuilder::new()
-		.with_current_project()
-		.import_memory()
-		.export_heap_base()
-		.build()
+	WasmBuilder::build_using_defaults();
 }