Remove Anyhow usage to make the crate's error handling `no_std` compatible

[CPerezz]

Managing error handling in no_std crates that rely on dusk-plonk it's quite tough since all of the errors use anyhow::Result and anyhow::Error. This is a problem since anyhow always gets inclued and it's no_std version forces to include allocators which might not be desired.

The idea is to create new types for the plonk errors that are no-std compatible and that impl display and other std-related stuff under a std flag.

[CPerezz]

You can implement Display now without the flag and we will add the flags once we introduce alloc and std features.

[burdges]

All true rust types know their size and alignment, which dyn Trait types accomplish using their vtable.

In consequence, one could build a TinyBox<T: Sized> type that allocated when size_of_val(&T) > size_of::<usize>() but otherwise stored T internally, and then TinyBox<dyn Trait> could actually work correctly by using size_of_val(&self), which only accesses the vtable, not the actual self data pointer.

We could then make TinyBox work even without alloc, simply by making its non-alloc form panic when size_of_val(&T) > size_of::<usize>().

At this point, if one has a no_std compatable Error trait then one could rebuild the entire trait infrastructure of std::io on top of this trait. This gives you Result<T,dyn Error> which works even without alloc for small enums, OS ernos, etc., but not large complex error types with backtrace allocations, etc.

I've zero time to implement this, but if anyone gets interested then I'm happy to give some pointers.

cc rust-lang/project-error-handling#20

[CPerezz]

I really like the idea @burdges . And also seems that there's work on the community towards a similar solution.
I'll add an issue pointing to this message so that we can investigate further once we have more time (or if someone wants to work on it).

Thanks a lot for the suggestion and taking time to explain it, is really interesting! 😃