R API for contexts

There should be an R api to create contexts. This needs some thinking as to ensure that the R implementation and the C++ code of contexts stay in sync. In R we can represent the context just as a RAW vector or similar. Probably the most part should be done in C++. One option would be to use envs?

function() {
  arg1 <- c("eager","int")
  arg3 <- c("eager")
  noReflection <- T
  new.context(environment())
}

This might collide with #913