Fixed evaluation of custom nodes with duplicate input names: MAGN-4436, MAGN-4885

When compiling the AST for custom node definitions, we were using the input names as-is. This means that, if two inputs were the same, we were compiling a DS function with duplicate parameter names. This was causing the VM to get confused (technically this behavior is undefined) and I believe it used the second input as both inputs.

This fix changes the custom node AST compilation so that all inputs are given unique identifiers, even if the names are the same.

#### Reviewers:
* [ ] @lukechurch 
* [ ] @ke-yu