src: add symbol range expression to further compact intervals

src: add symbol range expression to further compact intervals

Update parser to use a new symbol range expression with smaller memory
footprint than range expression + two symbol expressions.

The evaluation step translates this into EXPR_RANGE_VALUE for interval
sets.

Note that maps or concatenations still use the less compact range
expressions representation, those require more work to use this new
symbol range expression. The parser also uses the classic range
expression if variables are used.

Testing with a 100k intervals, worst case scenario: no prefix or
singleton elements. This shows a reduction from 49.58 Mbytes to
35.47 Mbytes (-29.56% memory footprint for this case).

This follow up work to previous commits:

 91dc281a82ea ("src: rework singleton interval transformation to reduce memory consumption")
 c9ee9032b0ee ("src: add EXPR_RANGE_VALUE expression and use it")

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>