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.

templates/minimal/runtime/build.rs

@@ -18,10 +18,6 @@
 fn main() {
 	#[cfg(feature = "std")]
 	{
-		substrate_wasm_builder::WasmBuilder::new()
-			.with_current_project()
-			.export_heap_base()
-			.import_memory()
-			.build();
+		substrate_wasm_builder::WasmBuilder::build_using_defaults();
 	}
 }