various improvements for XDR code generation

[bertschinger]

• Helpers: consider moving the helpers to a library that has to be included, rather than adding them into the generated code
  • benefits: deserializeError type doesn't get duplicated in each module, dead code warning isn't necessary for modules that don't use all the helpers
• Fix use std::os::unix::ffi::OsStrExt; being added for XDR schemas that don't need it (schema that has something like typedef string FileName<>; but no objects use FileName)
  • or just don't worry about this? Fix the schema?
• input parameter to deserialize method (signature like pub fn deserialize(&mut self, mut input: &mut &[u8]) -> Result<(), helpers::DeserializeError>: in some cases this needs to be mutable (mut input: &mut ...) and in some cases it doesn't. (input: &mut ...).
  • This leads to warnings if it's always marked as mutable when some methods don't need that. Better way to handle this?

[bertschinger]

fixed unnecessary mut in input parameter here: 97b1fb5