eemeir – Ad­just the gen­der of words in a doc­u­ment

De­fines macros for third-per­son sin­gu­lar pro­nouns (\E, \Em, \Eir, \Eirs), which ex­pand dif­fer­ently ac­cord­ing to a mas­cu­line/fem­i­nine switch. (If the switch is ‘mas­cu­line’, they would ex­pand to ‘he’, ‘him’, ‘his’ and ‘his’; if ‘fem­i­nine’, they would ex­pand to ‘she’, ‘her’, ‘her’ and ‘hers’. Apart from the pro­nouns, one can de­fine ‘word pairs’, such as mother/fa­ther, daugh­ter/son, and so on.
Gen­der may be de­fined once per doc­u­ment, as an en­vi­ron­ment, or may be flipped on the fly.