refactor: No Std support in zeth-lib & primitives

[CeciliaZ030]

This PR stipped off std from Zeth's lib and primitives packages which builds a block. The purpose is to allow Zeth to run on other ZkVMs which mostly requires no_std.

• add #![cfg_attr(not(feature = "std"), no_std)] in lib & primitives with default feature containing std
• move the lib/host module to host/src/util which means all RPC and file-caching utilities are with the host
• only dependency that doesn't support no_std is thiserror, hence replaced with thiserror-no-std, which requires some work arounds since we lost the blanket impl from std::error::Error

The core component revm now is also no_std with bluealloy/revm#987

[CeciliaZ030]

Hi @Wollac 😊 would appreciate if you take a look and consider merging it.
no-std is important for cross-platform support. Although Risc0 can do std, stripping off IO from lib makes the block builder cleaner.

[Wollac]

I think you are right that thiserror and std::io are the only real std requirements we have:

• thiserror-no-std doesn't seem to be the solution, as that crate doesn't seem to be well maintained and doesn't even have a readme. Probably implementing the errors without derive (i.e. implementing display and some from) is the much better solution.
• core2 is in even worse shape, so getting rid of all io would be required.
• Mapping errors just to be able to use context is useless, and they should be mapped directly into an anyhow.

stripping off IO from lib makes the block builder cleaner.

I am not sure I understand what you are saying.

In general, I have to admit that I am very skeptical about going no-std. std simplifies certain things significantly and has no real drawbacks on the R0 zkVM. I do believe that eventually other zkVMs will (have to) add std support.