Part of #700.

Currently, tarpaulin only runs tests against onefetch, but when it calculates test coverage, all source files including other packages are taken into consideration, which is why some files like ascii/src/lib.rs has 0 coverage even if test cases do exist. To fix this, tarpaulin needs an additional parameter --workspace to run tests in all packages in this repo.

Before:

$ cargo tarpaulin
...
|| Tested/Total Lines:
|| ascii/src/lib.rs: 0/119
...
54.87% coverage, 670/1221 lines covered

After:

$ cargo tarpaulin --workspace
...
|| Tested/Total Lines:
|| ascii/src/lib.rs: 90/119 +75.63%
...
63.06% coverage, 770/1221 lines covered, +8.19% change in coverage