Updated WGI specification to ease future migration to unicode support.
    Use STRING_TABLE, and better interface of READABLE_STRING_GENERAL,
    this way the signature are more flexible for unicode keys.

    Note that for now, unicode environment variables are not correctly supported in WGI
    especially the value of the variables.
    Any layer on top of EWGSI suffers from the same issues.

Better exception handling

+ code cleaning
Removed a few obsolete usages, and benefit from new classes from EiffelStudio >= 7.2