math/big,regexp: implement the encoding.TextAppender interface

For #62384

Change-Id: I1557704c6a0f9c6f3b9aad001374dd5cdbc99065
GitHub-Last-Rev: c258d18ccedab5feeb481a2431d5647bde7e5c58
GitHub-Pull-Request: golang/go#68893
Reviewed-on: https://go-review.googlesource.com/c/go/+/605758


Reviewed-by: Ian Lance Taylor <iant@google.com>
Commit-Queue: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Robert Griesemer <gri@google.com>