Require mbstring. Remove support for native php parser.

environment.xml

lang/en/qtype_stack.php

stack/answertest/at_general_cas.class.php

stack/answertest/atnumsigfigs.class.php

stack/cas/cassecurity.class.php

stack/cas/connector.class.php

stack/cas/parsingrules/002_log_candy.filter.php

.../parsingrules/090_special_forbidden_characters.filter.php

...les/402_split_prefix_from_common_function_name.filter.php

...rsingrules/403_split_at_number_letter_boundary.filter.php

...les/404_split_at_number_letter_number_boundary.filter.php

stack/cas/parsingrules/410_single_char_vars.filter.php

stack/cas/parsingrules/990_no_fixing_spaces.filter.php

stack/cas/parsingrules/991_no_fixing_stars.filter.php

stack/cas/parsingrules/998_security.filter.php

stack/cas/parsingrules/999_strict.filter.php

stack/maxima/stackmaxima.mac

stack/maximaparser/MP_classes.php

stack/maximaparser/autogen/gen.js

stack/maximaparser/autogen/notes.txt