Record update

[foxfriends]

Currently the semantics of record tails when the field is defined both in the head and the tail is not very clear. This should be made more clear, such that a field in both head and tail is valid, but the one in the head will take priority. This should help make updating a record a little easier:

// Currently, name must be removed from the record so it can be updated reliably. 
// This also requires the record previously had the name field.
setName(Name, person { name: _, ..P }, person { name: Name, ..P }).
// After change, this would not be the case:
setName(Name, person(P), person { name: Name, ..P }).