This was meant to support:

  env do |req|
    append_path 'PATH', req.some_method
    ...
  end

i.e., the block was evaluated in the context of ENV. But it turned out
to be not so useful after all, so I'm ripping it out before something
actually depends on it.

Not thread safe! But I don't think we care.

We want to evaluate the env DSL block in the context of ENV for asthetic
reasons, but we also want access to methods on the requirement instance.
We can use #instance_exec to pass the requirement itself into the block:

  class Foo < Requirement
    env do |req|
      append 'PATH', req.some_path
    end

    def some_path
      which 'something'
    end
  end

Also add a simplified version of Object#instance_exec for Ruby 1.8.6.

In addition to

  env :userpaths
  env :std

requirements can now do

  env do
    append 'PATH', '/some/path/to/bin'
    # and more
  end

Closes Homebrew/homebrew#14654.