We want that the runtime only depends on `FoundationEssentials` where
available (ie. on linux) to ensure small binary size.
### Motivation:
Smaller binary size is good for lambda deployment and cold-start times.
The runtime should only depend on `FoundationEssentials`.
### Modifications:
- replace `import Foundation` with `import FoundationEssentials` if
`FoundationEssentials` is available.
- I also applied the same treatment to tests to ensure that catch error
where tests run on linux and we use API that is only available in
`Foundation` which easily happens when you develop on macOS (where
always full `Foundation` is available).
### Result:
This should allow builds without linking full `Foundation`.