Build pipeline evolution

[AnotherDaniel]

... 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
• Adding necessary bits for cross-compiling to aarch64-unknown-linux-gnu to CI/CD #73 add x-compilation from x86 to arm, to go into our nighly workflow and mainly serve as a proof-point/canary which I'm not sure we'll ever see roll off the perch
• Add GitHub workflow for creating 3rd party dependency list #25 incorporate the EF Dash tool for code scanning
• add cross-compile support for all valid Android targets
• add doctest steps to check and nightly workflows (as nexttest doesn't do doctests atm)