Add support for older builds of the 6.2 compiler

[lorentey]

Rip out all traces of SwiftCompatibilitySpan availability; it isn't actually functional in 6.2.

Older compiler builds seem to struggle with slightly more complex mutating dependencies and Container protocol requirements that have defaults on RandomAccessContainer.

The tests are still passing, which is a good sign.

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• I've followed the coding style of the rest of the project.
• I've added tests covering all new code paths my change adds to the project (if appropriate).
• I've added benchmarks covering new functionality (if appropriate).
• I've verified that my change does not break any existing tests or introduce unexplained benchmark regressions.
• I've updated the documentation if necessary.