`Microsoft.Quantum.Core.Default` should be marked as deprecated and then removed

[swernli]

Likely a prerequisite: microsoft/QuantumLibraries#546

The Default function uses the deprecated and unsafe array creation syntax, and should not be supported any longer. We should mark the function as deprecated as well and prepare to remove it. This may require revisiting the libraries to ensure all remaining uses of it are removed as well.

Related: #977

[msoeken]

I want to bring attention to Cassandra Granade (@cgranade)'s comment microsoft/QuantumLibraries#553 (comment) in microsoft/QuantumLibraries#553. We have often used Default to initialize UDTs by partially or completely assigning UDT fields with the w/ syntax. Maybe it's worth to discuss an alternative to this paradigm as part of this issue.

[bamarsha]

Instead of Default<MyUdt>() w/ Field <- foo, you can use DefaultMyUdt() w/ Field <- foo by defining your own function:

function DefaultMyUdt() : MyUdt {
    return MyUdt(field1, field2, field3);
}