XmlAst.fs vs genericBackend.fs

The F# code of the XML and generic backends are duplicate with 3 minor differences (see below).

The XML backend is used when using -ast option in the command line, while genericBackend is used when using -customStgBackend.

Some tools (DMT) use -customStgBackend xml.stg instead of -ast because it allows to combine with -customStgAstVersion and specify which AST level is needed.

The problem is that each modification in one of the backend has to be duplicated in the other one to avoid regression. This does not make the maintenance and testing very easy.

I propose to remove the XmlAst.fs backend and make the tool use genericBackend.fs when using the -ast option.

Side note:
The differences beteween the two backends, which need a minor update are the following:
1) in the call to VasXml (Value assignment) the parameter passed to the XML backend is generated from PrintAsn1.PrintAsn1Value while the generic backend used a specific PrintCustomAsn1Value (which is better and needs to be reflected in the xml.stg file)

2) the generic backend has an "AssigOp" template which does not exist in the xml template and would need to be added

3) the XML backend does not render the Contracts associated to the types (the last parameter of TaxXml is an empty string, while the genericBackend calls PrintContract).

These 3 differences are minor, since as said above the xml.stg backend can work with the genericBackend, so the only thing to do is to make sure that the PrintCustomAsn1Value in xml.stg use the same formating as the one in PrintAsn1.