avoid ambiguous associated item in TryFrom implementations

[hunts]

This was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

See also: rust-lang/rust#57644

Without this change, latest versions of cargo would report this:

error: ambiguous associated item
   --> src/log_capnp.rs:213:69
    |
213 |             fn try_from(value: u16) -> ::core::result::Result<Self, Self::Error> {
    |                                                                     ^^^^^^^^^^^ help: use fully-qualified syntax: `<Level as TryFrom>::Error`
    |

[dwrensha]

Thanks! Fixed in 59f2ad3. (Your version didn't quite work.)