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.

substrate/utils/wasm-builder/src/lib.rs

@@ -33,15 +33,9 @@
 //! use substrate_wasm_builder::WasmBuilder;
 //!
 //! fn main() {
-//!     WasmBuilder::new()
-//!         // Tell the builder to build the project (crate) this `build.rs` is part of.
-//!         .with_current_project()
-//!         // Make sure to export the `heap_base` global, this is required by Substrate
-//!         .export_heap_base()
-//!         // Build the Wasm file so that it imports the memory (need to be provided by at instantiation)
-//!         .import_memory()
-//!         // Build it.
-//!         .build()
+//!     // Builds the WASM binary using the recommended defaults.
+//!     // If you need more control, you can call `new` or `init_with_defaults`.
+//!     WasmBuilder::build_using_defaults();
 //! }
 //! ```
 //!