fix: Fix FromStr not found error on AccountId20

[conr2d]

This PR fixes the following issue.

---

When building fp-account without default features with the command:

cargo build -p fp-account --no-default-features --target wasm32-unknown-unknown

an error will occur because core::fmt::FromStr is not implemented for H160 without default features.

    Checking fp-account v1.0.0-dev (/Users/conr2d/Projects/frontier/primitives/account)
error[E0599]: no function or associated item named `from_str` found for struct `H160` in the current scope
  --> primitives/account/src/lib.rs:51:9
   |
51 |         H160::from_str(s)
   |               ^^^^^^^^ function or associated item not found in `H160`
   |

[boundless-forest]

Normally, we don't compile the primitive crate using the command params that you pasted. Is there any specific reason behind this? I personally think this hack is not worth it.

[conr2d]

I am writing a crate to extend Substrate features including fp-account, but this prevents using fp-account in a no_std environment. Is the fp-account crate intended to be used exclusively with Frontier? According to #1218, there are plans to publish Frontier crates to crates.io. In my opinion, a reusable component should build successfully without default features.

I added the --target wasm32-unknown-unknown flag because the underlying Substrate dependency requires a pointer size of 4 bytes in a no_std environment. The use of core::fmt::FromStr instead of std::fmt::FromStr suggests that this implementation is intended for no_std use, but it currently doesn’t support that case correctly. The implementation should either be changed to use std::fmt::FromStr with a feature guard, such as #[cfg(feature = "std")] or corrected to properly handle no_std environments like this PR.

Moreover, I don't believe my change qualifies as a hack since it doesn't introduce any new dependencies or use unsafe or non-idiomatic syntax. :)