Support calling ToString on an arbitrary value

[robin-aws]

This functionality is already indirectly exposed through the print statement, which is documented as using "the System.Object.ToString() method to convert the values to printable strings" (

dafny/Docs/DafnyRef/DafnyRef.mdk

Lines 5939 to 5941 in 14d0af1

	list of expressions to the console. The generated C# code uses
	the `System.Object.ToString()` method to convert the values to printable
	strings. The expressions may of course include strings that are used

).

It would be useful to be able to call this with a similar built-in function that returns a string, for building strings from values without necessarily printing them to the console.