Rename context parameter to guarantee it doesn't capture any free variables

[benjamn]

The only new variable introduced in the scope of the inner generator function is the context or $ctx parameter, and while it might be acceptable to say "just don't name any variables context or $ctx," it would really be ideal to synthesize the name of the parameter such that we can be completely confident of its freshness.

The great thing about having the context parameter and knowing that it doesn't collide with any other variables is that the generated code can then add arbitrary properties to it without any concern for scope pollution.