Please add tests for the no_std variant

[dkg]

Looks like the mod tests sections in src/{encode,decode}_impl.rs both test the high-level interface, which requires std

that means that a test without the std feature will fail:

0 dkg@hos:…/libzbase32$ cargo --offline test --all-targets --no-default-features 
   Compiling libzbase32 v2.0.1 (…/libzbase32)
error[E0432]: unresolved import `super::decode`
   --> src/decode_impl.rs:264:9
    |
264 |     use super::decode;
    |         ^^^^^^^^^^^^^ no `decode` in `decode_impl`

error[E0432]: unresolved import `super::encode`
   --> src/encode_impl.rs:253:9
    |
253 |     use super::encode;
    |         ^^^^^^^^^^^^^ no `encode` in `encode_impl`

error[E0433]: failed to resolve: use of undeclared type `Vec`
   --> src/decode_impl.rs:268:26
    |
268 |         let mut buffer = Vec::new();
    |                          ^^^ use of undeclared type `Vec`

error[E0433]: failed to resolve: use of undeclared type `String`
   --> src/encode_impl.rs:257:26
    |
257 |         let mut buffer = String::new();
    |                          ^^^^^^ use of undeclared type `String`

Some errors have detailed explanations: E0432, E0433.
For more information about an error, try `rustc --explain E0432`.
error: could not compile `libzbase32` due to 4 previous errors
101 dkg@host:…/libzbase32$ 

If i decorate the two mod tests sections with #[cfg(feature = "std")] then i get a handful of warnings about unused items in src/test_data.rs -- and no tests run at all ☹

Seems like the right thing to do is to add tests for the low level api.