Replaced langMatches(a, b) with lang(a) = b

[floriankramer]

This fixes the standard conformance problems mentioned in #194. This pr does not implement any version of langMatches though.

[niklas88]

@floriankramer, @joka921 so I'm thinking we should add this implementation of lang(a) == b but also keep the current langMatches() until we have implemented it correctly. I know the current version breaks the spec but a) keeping it means we don't break existing queries b) our result for langMatches() is a subset of the correct result c) langMatches() is a pretty weird and in my opinion useless function.

[niklas88]

@floriankramer after readding the old langMatches() behavior, i've this running on Wikidata but it seems the syntax doesn't work. The old langMatches() still works fine but e.g. the following query returns empty results instead of German cities. I also tried the wrong == but that is correctly recognized as invalid syntax.

PREFIX wd: <http://www.wikidata.org/entity/>
PREFIX wdt: <http://www.wikidata.org/prop/direct/>
PREFIX schema: <http://schema.org/>
SELECT ?name ?populationWHERE {
  ?city wdt:P31 wd:Q515 .
  ?city wdt:P17 wd:Q183 .
  ?city wdt:P1082 ?population .
  ?city schema:name ?name .
  FILTER (lang(?name) = "de") .
}
ORDER BY DESC(?population)

[niklas88]

Not LGTM as I currenlt can't get this to work, would guess this is something small though

[floriankramer]

@niklas88 The code was trying to get the language tag from the = sign instead of the actual language tag. Should be fixed now.

[niklas88]

LGTM and tested