Avoid duplicate names when compiling arrow types

[atomb]

Description

Dafny previously could generate duplicate variable names when translating arrow types. For example, when compiling this file to Java with the following command:

dafny build --target:java --unicode-char:false JSONParser.dfy

How has this been tested?

Manual testing of the above command, plus a new test: comp/DuplicateArrowNames.dfy.

By submitting this pull request, I confirm that my contribution is made under the terms of the MIT license.

[keyboardDrummer]

Let's add a test before merging

[MikaelMayer]

I would love to merge it asap because it contains a hot fix for a user, but at the same time I understand that without testing, the fix might not be as robust as we are used to. Since this PR does not claim to fix any issue, could this be just considered code refactoring? Down the road we could make use of the upcoming parsers combinators library as a safeguard test.
Otherwise, of course, if Aaron can find a smaller test to reproduce the error, I would love it.

[fabiomadge]

Here's one that fails for Java:

method Main() {
  var f: nat -> nat -> bool;
  print f, "\n";
}

[atomb]

Here's one that fails for Java:

method Main() {
  var f: nat -> nat -> bool;
  print f, "\n";
}

Thanks! I'll add that one.

[fabiomadge]

Sorry for the late intervention.

[fabiomadge]

Suggested change

	print f, "\n";
	print f(0,1), "\n";

This way we get more uniform output.

[atomb]

Oh, yeah, I like that. :)