Rewrite xdrgen generator in Rust

[leighmcculloch]

What

Rewrite xdrgen generator in Rust, using templating.

Why

The existing xdrgen has evolved over time, but without a lot of thought and the lack of templating makes it extremely difficult to look at the code and understand the shape of the final generated code. It is also brittle, takes a long time to build and run, and that combination of brittle and slow to build makes iterating on it rather painful.

There's a handful of things we want to do that'll be easier to iterate on with a faster build and less brittle code base. For example:

• Add support for #ifdef #else #endif in .x files #497
• Make possible to build types with borrowed data instead of owned #454
• Deduplicate types across curr and next #467
• Make possible to build types with borrowed data instead of owned #454
• Change XDR to avoid need for ScVal::Object(Option<ScObject>) #133
• Add read/write capabilities when alloc feature disabled #47
• Add read/write capabilities to alloc feature when std not enabled #46
• Rust: Improve error messages when decoding xdrgen#200
• Rust: Generated comment on types has a typo xdrgen#129
• Expose .x comments in Rust code
• and other things