Inconsistent function names in core::result

[Dretch]

The is_success and is_failure functions/methods in core::result should be called is_ok and is_err instead, so that they correlate with the enum's tag names. These functions were originally called success / failure presumably to avoid clashing with the enum's tags (i.e. the functions could not be called ok / err), but now that they are prefixed with is_ then these additional names are not required.

An independent issue is that the documentation for is_failure (perhaps becoming is_err) should say "Returns true if the result is err" rather than "Returns true if the result is error" - because the enum tag is called err not error.

[brson]

Fixed by 2f060eb