Add shorthand `chain` syntax for custom matchers

The `chain` block in fluent matchers almost always just assigns its argument to an instance variable:

```ruby
RSpec::Matchers.define :be_bigger_than do |first|
  match do |actual|
    (actual > first) && (actual < @second)
  end

  chain :and_smaller_than do |second|
    @second = second
  end
end
```

Requiring a block is kind of overkill in this common case, and I get tired of reimplementing the exact same thing in every fluent matcher I define.

This pull request allows you to instead write:

```ruby
RSpec::Matchers.define :be_bigger_than do |first|
  match do |actual|
    (actual > first) && (actual < second)
  end

  chain :and_smaller_than, :second
end
```

Namely:

* you can just say `:second` instead of `do |second| @second = second end`; and
* as a bonus, since the DSL now knows a meaningful name for that remembered value, you can refer to it (e.g. in `match`) by saying `second` instead of `@second`.

I think this is an improvement. WDYT?
That’s a reasonable decision. Like @JonRowe I just felt bad using `instance_variable_set` rather than a writer, and there’s no pleasant way to make the accessors private (`private *attr_names.map { |attr_name| :"#{attr_name}=" }`? ugh) without generating a warning, so leaving them public was my compromise.

Assuming I remove the writers, would you also support making the readers private to reduce the public surface area of the matcher?