Check workspace members with default features individually in CI

Check workspace members with default features individually in CI

Previously, we would only check the workspace as a whole. This however
would mean that we would check/test crates with `lightning`'s default
features enabled, allowing failures-to-build under the crates own
default features to slip through, if they didn't explicitly enable
`lightning/std`, for example.

Here, we extend the CI to check the workspace as a whole but then run
checks, tests, and doc generation on the workspace members individually,
asserting that all of them build even when not built as part of the same
workspace as `lightning`.