Reduce to "basic optimizations" for the steven_blocks module, so it
doesn't take hours of time and gigabytes of memory to compile. The main
program and other code still builds with full optimizations in release
mode, to accomplish this, the profile-overrides feature is required so
we also switch to nightly Rust (to be switched to 1.41+ in #258).
* Update to rustc 1.42.0-nightly (760ce94c6 2020-01-04)
* Update builds.sr.ht to use +nightly
* Override opt-level=1 for steven_blocks using profile-overrides