`Cross` support tracking issue

[nia-e]

Currently, building via cross requires manual patching of this crate's Cross.toml. We need to figure out a durable, convenient way to allow for this.
Relevant issues and PRs so far, to both cross and to cross-compilation in general:

• cross compiling #53
• Basic cross infrastructure and fixups #72
• Automatic tests for non-x86 compile #73