cumulus-test-relay-sproof-builder doesn't build for wasm

[xlc]

cargo check -p cumulus-test-relay-sproof-builder --no-default-features --target=wasm32-unknown-unknown

error[E0152]: found duplicate lang item `panic_impl`
    --> /Users/xiliangchen/.cargo/git/checkouts/substrate-7e08433d4c370a21/4e1e17c/primitives/io/src/lib.rs:1720:1
     |
1720 | pub fn panic(info: &core::panic::PanicInfo) -> ! {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: the lang item is first defined in crate `std` (which `parity_scale_codec` depends on)
     = note: first definition in `std` loaded from /Users/xiliangchen/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/wasm32-unknown-unknown/lib/libstd-a2fd8ecf580522db.rlib
     = note: second definition in the local crate (`sp_io`)

error[E0152]: found duplicate lang item `oom`
    --> /Users/xiliangchen/.cargo/git/checkouts/substrate-7e08433d4c370a21/4e1e17c/primitives/io/src/lib.rs:1736:1
     |
1736 | pub fn oom(_: core::alloc::Layout) -> ! {
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     |
     = note: the lang item is first defined in crate `std` (which `parity_scale_codec` depends on)
     = note: first definition in `std` loaded from /Users/xiliangchen/.rustup/toolchains/nightly-aarch64-apple-darwin/lib/rustlib/wasm32-unknown-unknown/lib/libstd-a2fd8ecf580522db.rlib
     = note: second definition in the local crate (`sp_io`)

For more information about this error, try `rustc --explain E0152`.
error: could not compile `sp-io` due to 2 previous errors

[bkchr]

This crate wasn't build for no_std. I see that we have there an std feature, but it is misleading. From the Substrate side we don't have any support for recording a proof right now in no_std. We could switch to some the "bare" trie primitives to achieve this, but it would require some more changes.