Remove extraneous Input::Error trait bounds

[softmoth]

Due to an old Rust bug, rust-lang/rust#24159, many parsers included an extraneous trait bound for Input::Error as a workaround. That bug has been closed since February 2021, and the workaround is no longer needed.

There are a few places I did NOT remove Input::Error bound from the where clause, which would be nice to eliminate but I think may be required:

• stream/position.rs requires it to specify the StreamError associated type in the Positioned and StreamOnce implementations.
• stream/easy.rs similarly requires it to specify StreamError in the docs example
• parser/combinator.rs requires Into<...::StreamError> for the error generic in its AndThen struct
• src/lib.rs has similar awkward boilerplate for ...:StreamError: From<ParseIntError> in its docs examples

[Marwes]

LGTM, but tests are failing due to testing on an old rust. You can bump that to the version that has the fix and I can merge.