Introduce as_data in prelude

[KtorZ]

New prelude function to conveniently upcast any serialisable type into Data in places where the compiler cannot do it implicitly.

See also: aiken-lang/prelude#11

Note

I fancied for a second introducing a new language syntax for this, like:

 type Foo {
     foo: Int
 }
 type Bar {
    bar: Int
 }

 test foo() {
   [~Foo(14), ~Bar(42)] != []
 }

But that felt a bit overkill on top of making the language more complex. The as_data function is already fully erase at build time, so it has the same effects anyway.