Remove std dependency for Error and Display trait implementations

[jbis9051]

strum uses std::error::Error and std::fmt::Display instead of core::error::Error and core::fmt::Display:

strum/strum/src/lib.rs

Line 58 in c8d6241

impl std::error::Error for ParseError {

strum/strum/src/lib.rs

Line 47 in c8d6241

impl std::fmt::Display for ParseError {

This makes it impossible to use these traits in no_std environments.