Comparison doesn't handle funs

By removing the `default` switch case I get the following warning: `term.c:566:17: warning: enumeration value ‘TERM_TYPE_INDEX_FUNCTION’ not handled in switch [-Wswitch]` that likely means we are not handling comparisons with funs.

TERM_TYPE_INDEX_FUNCTION must be handled in `term_compare()` implementation in term.c.

Also fun comparison must be added to our tests.