`no_std` + `alloc` support

Would it be possible to add an option for the generated code to avoid referencing `std` and imports that are only in the prelude when `std` is linked, e.g. `std::result::Result` and `ToString`, and instead use e.g. `core::result::Result` and `alloc::string::ToString`?

No files or tests are named. Begin by locating the generator code that emits std::result::Result and ToString, then identify how generation options are configured; done means the new option produces code using core and alloc without std references, with coverage for the option.

Written by the indexing model from the issue text.