RegexpTransformer: use internal buffering

[icholy]

The current replace.RegexpTransformer offloads buffering to the calling code. It does this by repeatedly returning transform.ErrShortSrc until enough data is available. Even though this technically conforms to the documentation/contract, it fails when used with transform.Chain due to its internal (undocumented) 2kb buffering limit #5 golang/go#49117.

The replace.Chain was added as an alternative to transform.Chain. However, each call to Chain allocates 8kb for each underlying transform.Reader. I've opened a proposal to add a transform.Reader.Reset method, but the author of x/text is unresponsive.

We can avoid all these problems by doing the buffering ourselves.