Remove workspace_members proc macro

[macladson]

Issue Addressed

I noticed that the logging crate was often being spuriously recompiled due to the workspace_crates macro running even when no changes needed to be made.

Proposed Changes

This removes the proc macro and instead uses a build script to construct the list of workspace members and save them to a file in logging's OUT_DIR. This build script should only be rerun when actually necessary which should reduce spurious recompiles.

Additional Info

The build script will be rerun when:

1. This build.rs file is changed.
2. The workspace Cargo.toml changes.
3. Build dependencies change.