Add errors stabilized in 1.34 to the core Error shims

[shepmaster]

Fixes #324

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[FedericoPonzi]

Thanks a lot for the quick pr! Unfortunately I cannot test it because it's not building, I'm getting the same error from the CI

error: no rules expected the token `core`
  --> src/no_std_error.rs:37:5
   |
17 | macro_rules! impl_error {
   | ----------------------- when calling this macro
...
37 |     core::array::TryFromSliceError, // 1.34
   |     ^^^^ no rules expected this token in macro call

error: could not compile `snafu` due to previous error

Exit status: 101

[shepmaster]

because it's not building

Well, that's embarrassing. Let's try that again.