* refactor: move toCachedNamedNode (private)
* chore: only NamedNodes predicates in removes
* feat: enforce NamedNode predicates in most cases
* feat: getAll only accepts NamedNodes
* feat: toCachedNamedNode only accepts string arg
* tests: use NamedNodes for getAll calls
* test: remove unnecessary string check for coverage
* tests: fix NamedNodes in new tests after rebase
* feat: metadatawriters store NamedNodes
* refactor: toCachedNamedNode as utility function
* fix: double write of linkRelMap
* test: use the CONTENT_TYPE constant