Compile the wasm runtime with a stable toolchain

[pepyakin]

We require nightly compiler for building the wasm runtime. The main reason for that is several important features was available only on the nightly compilers.

However, as the time progresses more and more features are promoted to stable.

• alloc crate (tracking issue, RFC, PR)
• std::arch::wasm32::unreachable (tracking issue)
• core_intrinsics (trie/hash-db, not sure why)
• waiting on alloc_error_handler becoming panic by default (tracking issue, formelly this)

[pepyakin]

The alloc crate is on the course of stabilization: PR

[gui1117]

alloc crate is stabilized by the way

[tomaka]

The biggest offenders for this are actually not in Substrate, but dependencies such as elastic-array or hash-db.
There seems to be way more than just std::arch::wasm32::unreachable missing.

[pepyakin]

• One dependency that still uses features is safe_mix. It does so needlessly and actually is already fixed on master (not published yet though).
• core::arch::wasm32::unreachable was landed and it allowed us to get rid of core_intrinsics, see Get rid of in-substrate usages of core_intrinsics feature #4823.
• alloc_handler_error is a real problem. We cannot work it around since liballoc requires its presence if used. There are some activity in discussion with some unhappy splats coming out of it, so 🤷‍♂