Term api and io world

[AdrienChampion]

• remove the obsolete (void) ioWorld argument in FFI (see here and here): simplifies the env-catch-macro and env/error-handling in general

• add all Term features from the C++ API except toStringValue, which requires some discussion because lean strings support unicode

  also, some Term functions were modified to account for errors, usually on null terms: mainly

  • getNumChildren, which now produces 0 on null
  • getSort, getKind, and getId now produce Except Error _ results (with ! and ? variants) to deal with null terms

• add all Term unit tests except toStringValue-related tests

• improve test syntax extensions slightly