Add cross-testing workflow #108

Job	Run time
Test - Stable Toolchain + Recent Lockfile	1m 21s
Test - Stable Toolchain + Minimal Lockfile	3m 14s
Test - MSRV Toolchain + Minimal Lockfile	3m 38s
Check - Check Formatting	49s
Check - Build Documentation	1m 30s
Check - Lint	1m 40s
	12m 12s

Provide feedback