math/big: Fix variable name in documentation for *Int.Bytes and BitLen.

R=golang-dev, iant, gri
CC=golang-dev
https://codereview.appspot.com/13869043

Committer: Robert Griesemer <gri@golang.org>
go/ast: add Slice3 field to SliceExpr

If Slice3 is set, the expression is
a 3-index slice expression (2 colons).
Required for type-checking.

Backward-compatible API extension.

R=r, rsc
CC=golang-dev
https://codereview.appspot.com/13826050