Subxt Metadata: #[no_std] compatibility (#1401)

* no-std tests and porting of subxt-metadata

* update pipeline

* fix generate custom metadata test

* fix cargo run command

* adjust pipeline

* remove prelude from subxt-metadata

* revert autoformatting of Cargo.toml

* remove alloc::format! again, still causes linker errors

* add no-std-build for thumbv7em-none-eabi target

* remove std feature flag

* remove libc and add small readme with test instructions

* change ci for nightly no std

testing/no-std-tests/src/main.rs

@@ -0,0 +1,47 @@
+// Copyright 2019-2023 Parity Technologies (UK) Ltd.
+// This file is dual-licensed as Apache-2.0 or GPL-3.0.
+// see LICENSE for license details.
+
+#![allow(internal_features)]
+#![feature(lang_items, start)]
+#![feature(alloc_error_handler)]
+#![no_std]
+
+#[start]
+fn start(_argc: isize, _argv: *const *const u8) -> isize {
+    compile_test();
+    0
+}
+
+#[lang = "eh_personality"]
+#[no_mangle]
+pub extern "C" fn rust_eh_personality() {}
+
+#[panic_handler]
+fn panic(_info: &core::panic::PanicInfo) -> ! {
+    loop {}
+}
+
+use libc_alloc::LibcAlloc;
+
+#[global_allocator]
+static ALLOCATOR: LibcAlloc = LibcAlloc;
+
+//////////////////////////////////////////////////////////////////////////////
+
+extern crate alloc;
+
+/// Including code here makes sure it is not pruned.
+/// We want all code included to compile fine for the `thumbv7em-none-eabi` target.
+fn compile_test() {
+    subxt_metadata_compiles();
+}
+
+fn subxt_metadata_compiles() {
+    use codec::Decode;
+    let bytes: alloc::vec::Vec<u8> = alloc::vec![0, 1, 2, 3, 4];
+    subxt_metadata::Metadata::decode(&mut &bytes[..]).expect_err("invalid byte sequence");
+
+    const METADATA: &[u8] = include_bytes!("../../../artifacts/polkadot_metadata_small.scale");
+    subxt_metadata::Metadata::decode(&mut &METADATA[..]).expect("should be valid metadata");
+}