chore: fix cargo clippy warning

[xixishidibei]

Running cargo clippy reports the following error.

warning: calling `as_bytes` after slicing a string
   --> src/primitives/decode.rs:138:30
    |
138 |             data_part_ascii: rest[1..].as_bytes(), // Skip the separator.
    |                              ^^^^^^^^^^^^^^^^^^^^ help: try: `&rest.as_bytes()[1..]`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#sliced_string_as_bytes
    = note: `#[warn(clippy::sliced_string_as_bytes)]` on by default

This commit resolves the issue.

[apoelstra]

CI failures are unrelated.

[apoelstra]

ACK 9390a63; successfully ran local tests

[clarkmoody]

ACK 9390a63