Warn about unconditional inject-at-head-cancel mixins

[i509VCB]

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This can be made with an Injector mixin that always cancels the rest of the function (at head) and possibly returns a different value.

This is effectively an overwrite with undefined behavior.

1. The ordering is undefined unless you change mixin priority. Two of these could conflict
2. Any transformations afterwards are entirely ignored. An overwrite would be more explicit about this behavior.

If an inject-at-head injector has a conditional that can change (unfamiliar with control flow in PSI, may not be feasible), then said warning should not be risen.

[Earthcomputer]

This should be fairly easy to implement.