data: Added sdb_data_expr_eval().

This function may be used to evaluate simple operation expressions on data
values. Supported operators are +, -, *, /, %, and (for string and binary
values) concatenation.
store_expr: Added framework for arithmetic expressions.

A store expression describes the parse tree of an arithmetic expression and
provides means to execute it (based on sdb_data_expr_eval()).
frontend: Added a parser for arithmetic expressions.

For now, the expressions are used in place of a datum but the parser will
actually evaluate the expression before passing it on to the store during
lookup. Since expressions currently cover constant results only, this is not a
limitation for now.