Right now many helpers have to deal with two modes of operation to
capture view output.

The main one is to swap the `@output_buffer` variable with a new buffer.
But since some view implementations such as `builder` keep a reference
on the buffer they were initialized with, this doesn't always work.

So additionally, the various capturing helpers also record the buffer
length prior to executing the block, and then `slice!` the buffer back
to its original size.

This is wasteful and make the code rather unclear.

Now that `OutputBuffer` is a delegator, I'd like to refactor all this
so that:

  - @output_buffer is no longer re-assigned
  - A single OutputBuffer instance is used for the entire response rendering
  - Instead capturing is done through `OutputBuffer#capture`

Once the above is achieved, it should allow us to enabled Erubi's
`:chain_appends` option and get some reduced template size and some
performance.

Not re-assigning `@output_buffer` will also allow template to access
the local variable instead of an instance variable, which is cheaper.

But more importantly, that should make the code easier to understand
and easier to be compatible with `StreamingBuffer`.