Add support for the use of `..` in structs

# Motivation

Currently, when a `struct` is instantiated every field must be specified.

As a user I may not care about every field when creating a new `struct` and thus it would be nice to have a default value take place for the fields that are not specified.

This would be done using the `..` syntax similar to `Rust`.

```rust
struct MyStruct {
field1: type1,
field2: type2,
field3: type3,
}

let my_struct = MyStruct { field2: value2, .. };
```

Related: #1568

Start by reviewing related issue #1568 and the compiler's existing struct-instantiation handling. The issue names no files or tests; done would require agreed semantics and support for the proposed `..` syntax so unspecified fields receive default values.

Written by the indexing model from the issue text.