Start consolidating more nodes in "single_node"

Been struggling lately with the responsibilities (and differences) of

single_node
call_expression
expr

as new rules have been added. Merging call expressions into single_node feels
like the right thing to do, but I'm noticing now with the rubyspec that "if"
statements are more fragile during parsing. My gut feeling right now is that
this is related to zero precedence rules between tokens / operators having
been defined yet, but I can't prove that without taking time for more research.

Until then ... you can now assign the result of a ternary expression.