Add a std feature for toggling no_std support

[jwodder]

• The library currently uses the following features that are not in core:

  • The Error trait (but see Move Error trait into core rust-lang/project-error-handling#3)
    • thiserror will have to be dropped as a dependency
  • str.to_ascii_lowercase()
  • std::time::{SystemTime, UNIX_EPOCH}
  • Anything else?

• The binary will require std.