Merge user and workspace settings (#3462)
## Summary
This PR follows Cargo's strategy for merging configuration, albeit in a
more limited way (we don't support as many configuration locations).
Specifically, we merge the user configuration with the workspace
configuration if both are present. The workspace configuration has
priority, such that we take values from the workspace configuration and
ignore those in the user configuration if both are specified for a given
setting -- with the exception of arrays and maps, which are
concatenated.
For now, if a user provides a configuration file with `--config-file`,
we _don't_ merge in the user settings.
See:
https://doc.rust-lang.org/cargo/reference/config.html#hierarchical-structure.
Closes #3420.
