fix compilation error on newer versions of rust

[alexanderkjall]

This fixes the following compilation error:

error[E0170]: pattern binding `a` is named the same as one of the variants of the type `ascii_char::AsciiChar`
   --> src/ascii_char.rs:797:21
    |
797 |             fn from(a: AsciiChar) -> $wider {
    |                     ^
...
827 | impl_into_partial_eq_ord! {u8, AsciiChar::as_byte}
    | -------------------------------------------------- in this macro invocation
    |
    = note: `#[deny(bindings_with_variant_name)]` on by default
    = note: this error originates in the macro `impl_into_partial_eq_ord` (in Nightly builds, run with -Z macro-backtrace for more info)

error[E0170]: pattern binding `a` is named the same as one of the variants of the type `ascii_char::AsciiChar`
   --> src/ascii_char.rs:797:21
    |
797 |             fn from(a: AsciiChar) -> $wider {
    |                     ^
...
828 | impl_into_partial_eq_ord! {char, AsciiChar::as_char}
    | ---------------------------------------------------- in this macro invocation
    |
    = note: this error originates in the macro `impl_into_partial_eq_ord` (in Nightly builds, run with -Z macro-backtrace for more info)