Build pipeline evolution

... to collect next steps in the evolution of our build pipeline and associated workflows... (and also so we can close #51 )

- [ ] review the scripts in ./tools, ensure we have locally-runnable versions especially of the check workflow
- [ ] #73 add x-compilation from x86 to arm, to go into our nightly workflow and mainly serve as a proof-point/canary which I'm not sure we'll ever see roll off the perch
- [X] #25 incorporate the EF Dash tool for code scanning
- [ ] add cross-compile support for [all valid Android targets](https://doc.rust-lang.org/rustc/platform-support/android.html)
- [X] add doctest steps to check and nightly workflows (as nexttest doesn't do doctests atm)