Added task-result-combinations strategy that creates all result combinations of given term.
Use task-result-combinations in PropLookup.
Merge branch 'better-proplookup'

Conflicts:
	org.metaborg.runtime.task/src/main/java/org/metaborg/runtime/task/util/CarthesianProduct.java
	org.spoofax.meta.runtime.libraries/task/core.str
	org.spoofax.meta.runtime.libraries/types/tasks.str