Sovereign SDK should work with stable rust

[bkolad]

Overview
Currently, we use nightly because we need an associated type default in the Module trait.
Once we have macro for deriving the Module trait automatically we will be able to move to stable rust.

Acceptance Criteria
Add a stable build to CI. Build passes.

[preston-evans98]

Moving to stable will make our CI build caching (#88) much more effective. You can't re-use dependency builds across compiler versions, and nightly updates every night - so our cache is effectively wiped every morning.

[citizen-stig]

As for today, I've checked what unstable features we use:

▸ grep -rh --exclude-dir=target '^#\!\[feature' .
#![feature(log_syntax)]
#![feature(associated_type_defaults)]
#![feature(array_windows)]
#![feature(array_chunks)]

1. associated_type_defaults Tracking issue for RFC 2532, "Associated type defaults" rust-lang/rust#29661
2. log_syntax Tracking issue for log_syntax, trace_macros rust-lang/rust#29598
3. array_windows Tracking Issue for slice::array_windows rust-lang/rust#75027
4. array_chunks Tracking Issue for slice::array_chunks rust-lang/rust#74985

So switching to stable rust will be possible only after these features are stabilitzed.

[citizen-stig]

Fixed by @preston-evans98 in #869