fstar! macro specialized for booleans

[W95Psp]

Currently we have:

• hax_lib::fstar! that returns unit (computationally irrelevant F* code)
• fstar! in the context of a computationally irrelevant code (e.g. in a req/ens clause), that can return any type

We can't have a polymorphic fstar! accessible in any context, because we'd need to return any type, that is we'd need to panic.

For now, we would like to have a fstar_bool!, a specialized, safe version of fstar!.
We could also have an polymorphicfstar_or_default! over types T: Default, that is defined as T::default().