Metal 7.1.0

This release provides some small changes/additions:

1. A new implementation of ConcatenatedValueSource, in order to resolve some runtime performance issues that were introduced in the previous (7.0.0) release.
2. Related to the previous change is an additional variant of the Cat ValueExpression that takes a single operand and then folds all the resulting values after evaluation.
3. The result of ValueExpressions used as a source by a Tie token are now cached, in order to prevent complex operations (e.g., decompressing data) from being performed repeatedly.
4. The behaviour of the Pre token before the 6.0.0 release is still often used in Metal descriptions, so this has been added to the Shorthand with the more precise name when.

For a more detailed description, please see the 7.1.0 Milestone.