Decompose strict equalities with Values nodes

[bcogrel]

This PR introduces decomposition of strict equalities for values node.

For instance, values nodes often contain IRI strings and they are joining with mapping definitions that builds these IRIs from an IRI template. Values nodes can be simplified to contain the values used as parameters in the IRI template instead of the IRI strings.

Similarly they should use natural DB values (e.g. integer) instead of strings to make the joins more efficient (with no cast-to-string anymore).

Example:

JOIN
   VALUES [x] ("http://localhost/thing/1/3"^^STRING) ("http://localhost/thing/2/4"^^STRING) ("http://localhost/thing/notANumber/5"^^STRING) (NULL) ("http://localhost/somethingelse"^^STRING)
   CONSTRUCT [x] [x/http://localhost/thing/{}/{}(LARGE_INTToSTRING(a),b)]
      EXTENSIONAL STR_TABLE1AR2(0:a,1:b)

into

CONSTRUCT [x] [x/http://localhost/thing/{}/{}(LARGE_INTToSTRING(a),b)]
   JOIN
      VALUES [b, a] ("3"^^STRING,"1"^^LARGE_INT) ("4"^^STRING,"2"^^LARGE_INT)
      EXTENSIONAL STR_TABLE1AR2(0:a,1:b)