DMN15 — Friendly enough cohersion to string

Currently concatenating strings is not intuitive to users, as I always have to convert non string values to strings using the built-in `string` function.

So

```
"Today is " + now() = null
```

In order to yield the actual string you explicitly convert `now()` to its string representation:

```
"Today is " + string(now()) = "Today is 2021-12-12"
```

A more user friendly behavior would be to coherse the second argument to string if the first argument is already a string. This always yields a string representation of the thing. If it is not what I desire I can use custom stringification mechanisms explicitly.

We'd need to amend the Table 57 (page 128, https://www.omg.org/spec/DMN/1.4/Beta1/PDF) to support such cohersion.

—

Additional details on this case can be found in https://github.com/dmn-tck/tck/issues/538 (DMN TCK issue on this topic).

Defer to DMN 1.6

Defer to DMN 1.6