Rewrote ci.yml workflow to parallelize steps

[pmarkowsky]

Build and test steps were taking more than 5 minutes to run so this cleans up
the workflow and parallelizes it.

It splits the workflow into the following 5 jobs.

Job Name	Depends on	What does it do?
preqs	N/A	Checks which files were modified in the PR and sets outputs for conditional execution of later jobs.
build_userspace	preqs	Ensures that any changes still result in a working build
build_driver	preqs	Ensures that any changes to the driver still build
unit_tests	preqs	Runs all unit tests
test_coverage	preqs	Runs all tests for coverage and uploads to Coveralls
preqs, build_userspace, build_driver, unit_tests, and test_coverage. The last four jobs depend on preqs.