Implement `SourceMapSource.buffer` for efficient size computation

[JoostK]

The Source.size method computes the size from the buffer length, where the default Source.buffer method allocates a new buffer from the string source. By implementing SourceMapSource.buffer this allocation is avoided, as a buffer representation is available on the instance. This results in a noticeable performance improvement for stats computation in Webpack, where chunk sizes are obtained from Source.size.

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[sokra]

Thanks