Invalid import of function via decode::{self} in tests

[Nemo157]

Useful error message from 1.27:

error: `self` no longer imports values
   --> bs58-rs/src/decode.rs:150:19
    |
150 |     use decode::{ self, DecodeError };
    |                   ^^^^
    |
    = note: #[deny(legacy_imports)] on by default
    = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
    = note: for more information, see issue #38260 <https://github.com/rust-lang/rust/issues/38260>

From 1.28 onwards this specific error disappears and instead there's just an error about the function decode not being found.

cc https://github.com/rust-lang/rust/issues/51845

[Nemo157]

Fixed by #10