Fix: new std feature to handle std error trait

[h4sh3d]

Follow up to #39

no_std should be opt out. Before releasing 1.1.0 I didn't realize we would break compatibility with all dependent crates because std::error::Error implemented through thiserror was removed. core::error::Error is nightly for now so we can't make it work without this new std feature. For now I prefer using thiserror to handle derivation for us, it's common and 1 liner instead of manually keeping the implementation of Error::source.

Today I'm wondering if 1.1.1 is ok, but breaking dependents with: { version = "1", default-features = false } or if 2.0.0 is necessary. For the later I'd then remove check from default as this was a bad idea from the beginning.

@mangoplane would you check that we are equivalent in a no_std environment as in #39

[codecov]

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Files Changed	Coverage
src/base58.rs	ø

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[kayabaNerve]

Bah. I made #44 and only then saw this, sorry.

I'd vote to default feature flag std. With std, it relies on std::error::Error. When core::error::Error is stabilized, the std feature can be dropped.