[Rigid/UniqueArray] Eliminate some duplicate bounds checks for UniqueArray

[Azoy]

UniqueArray had calls to _ensureFreeCapacity followed by calls to various RigidArray.append(*) which had its own bounds check. Split off the append logic from RigidArray into unchecked variants that it and UniqueArray can both call once they've done their bounds checking.

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.

[lorentey]

👍 Does the optimizer not fold away the second check? (Are we tracking that issue?)