Align search path lookup to standard semantics

[brodieG]

Don't start at top level with each substitution. So, if we find a symbol at certain level in the search path, we should only look for symbols further down the search path.

Not 100% sure this is the right thing to do, but I think this is necessary to conform to standard R semantics. That said, this is somewhat debatable since there is no real analog with normal expressions. Since this will be a real pain to implement (and will likely be slow) would should really make sure it is needed.

Started to think about the normal eval comparable:

env1 <- new.env()
env2 <- new.env(parent=env1)

env2$x <- 2

But then stopped when I realized that normal evaluation only substitutes a symbol once so what should happen on recursive evaluation is not really defined.