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

metadata/src/utils/variant_index.rs

@@ -2,8 +2,10 @@
 // This file is dual-licensed as Apache-2.0 or GPL-3.0.
 // see LICENSE for license details.
 
+use alloc::borrow::ToOwned;
+use alloc::string::String;
+use hashbrown::HashMap;
 use scale_info::{form::PortableForm, PortableRegistry, TypeDef, Variant};
-use std::collections::HashMap;
 
 /// Given some type ID and type registry, build a couple of
 /// indexes to look up variants by index or name. If the ID provided
@@ -62,8 +64,8 @@ impl VariantIndex {
         types: &'a PortableRegistry,
     ) -> Option<&'a Variant<PortableForm>>
     where
-        String: std::borrow::Borrow<K>,
-        K: std::hash::Hash + Eq + ?Sized,
+        String: alloc::borrow::Borrow<K>,
+        K: core::hash::Hash + Eq + ?Sized,
     {
         let pos = *self.by_name.get(name)?;
         let variants = Self::get(variant_id, types)?;