Adding procedural macros broke coverage runs

[dan-fritchman]

Adding the procedural-macros crate has caused all coverage runs since https://github.com/HW21/Spice21/actions/runs/441111829 to fail.

Apparently a known thing with the coverage runner (Tarpaulin): https://github.com/xd009642/tarpaulin#procedural-macros. The associated runtime-macros https://crates.io/crates/runtime-macros crate currently does not support derive macros.