`no_std` + `alloc` support

[tony-iqlusion]

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?

[tony-iqlusion]

Here's a fork that already supports this /cc @romac

https://github.com/informalsystems/pbjson

[Luctins]

Bump.

Any update on this?