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.

cumulus/test/runtime/build.rs

@@ -18,16 +18,10 @@
 fn main() {
 	use substrate_wasm_builder::WasmBuilder;
 
-	WasmBuilder::new()
-		.with_current_project()
-		.export_heap_base()
-		.import_memory()
-		.build();
+	WasmBuilder::build_using_defaults();
 
-	WasmBuilder::new()
-		.with_current_project()
+	WasmBuilder::init_with_defaults()
 		.enable_feature("increment-spec-version")
-		.import_memory()
 		.set_file_name("wasm_binary_spec_version_incremented.rs")
 		.build();
 }