impl ::core::error::Error instead of ::std::error::Error

[Lex3333]

::httparse::Error impls ::std::error::Error under the std feature flag when it could impl ::core::error::Error without a feature flag, and support errors without std flag.

Only downside is that this could potentially require MSRV bump - I don't know what the current MSRV is but ::core::error::Error was stabilized in 1.81.0.

[Lex3333]

This also appears, by a quick and possibly insufficient grep, to be the only use of the std feature in the whole crate. So, with this switch, the crate could become unconditionally no_std.