More convenient way to specify an annotation value with only one field

[niveathika]

Description:
Currently, the annotation should be a subtype of one of the three types: true, mapvalue:Cloneable, mapvalue:Cloneanble[].

When annotations are used for mapping record fields such as JSON to/from record conversion and database query results mapping, the annotation needs to contain only one value. Here, repeating the key for the value is a bit cumbersome since there will be numerous fields.

type Person record {
    int id;
    @sql:Column { name = "first_name" }
    string firstName;
    @sql:Column { name = "last_name" }
    string lastName
};

Here, the name key needs to be repeated again and again. With support for string type, this can be simplified as follows:

type Person record {
    int id;
    @sql:Column { "first_name" }
    string firstName;
    @sql:Column { "last_name" }
    string lastName
};

[jclark]

Two ways to approach this:

1. The value of an annotation is a string rather than a record. (In this case, there would be no curly braces: just @sql:Column "firstName")
2. The field name can be defaulted (problem is that this conflicts with { x } meaning { x: x }