Investigate whether we should deprecate `Complex` and other types in favor of `swift-numerics`

[`swift-numerics`](https://github.com/apple/swift-numerics) seems to be the official implementation of these types, so to improve compatibility with the wider package ecosystem, we may want to consider deprecating these types.

Compare the existing Complex and other affected types with the corresponding implementations in swift-numerics. Determine whether deprecation would improve compatibility without leaving a clear migration path, and document a decision about whether these types should be deprecated.

Written by the indexing model from the issue text.