bufio: make Reader naming consistent

All the readers are denoted as `b` while for `Reader.Size()` it is `r`.

Change-Id: Ib6f97306c11b3abb2ff30edbc9f9362cad36d080
GitHub-Last-Rev: 992f88b374b5a309303b7fa1622ee629d0fb741b
GitHub-Pull-Request: golang/go#26205
Reviewed-on: https://go-review.googlesource.com/122156Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/bufio/bufio.go

@@ -63,7 +63,7 @@ func NewReader(rd io.Reader) *Reader {
 }
 
 // Size returns the size of the underlying buffer in bytes.
-func (r *Reader) Size() int { return len(r.buf) }
+func (b *Reader) Size() int { return len(b.buf) }
 
 // Reset discards any buffered data, resets all state, and switches
 // the buffered reader to read from r.