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/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "subxt-core-no-std-tests"
+edition = "2021"
+publish = false
+version = "0.0.0"
+resolver = "2"
+
+[dependencies]
+subxt-metadata = { path = "../../metadata", default-features = false }
+codec = { package = "parity-scale-codec", version = "3.6.9", default-features = false, features = ["derive"] }
+libc_alloc = { version = "1.0.6" }
+
+[profile.dev]
+panic = "abort"
+
+[profile.release]
+panic = "abort"
+
+# this shouldn't be needed, it's in workspace.exclude, but still
+# I get the complaint unless I add it...
+[workspace]