frontend/parser: Don't override fields of unknown types.
frontend/grammar: Added a destructor for data objects.
frontend/parser: Made clear that parser modes are flags.

It doesn't make a difference so far, but will avoid problems in the future.
Added tests for sdb_fe_parse_expr() and fixed the implementation.

That is, fixed a typo preventing the parsed expression from being returned and
fixed memory leaks in the matcher and expression parsers.
parser_test: Added various arithmetic expressions to the expr parser test.
store, frontend: Make IS (NOT) NULL an unary operator on expressions.

This is more flexible and in-line with all other operators being migrated to
expression-based constructs.