math/big: support new octal prefixes 0o and 0O

This CL extends the various SetString and Parse methods for
Ints, Rats, and Floats to accept the new octal prefixes.

The main change is in natconv.go, all other changes are
documentation and test updates.

Finally, this CL also fixes TestRatSetString which silently
dropped certain failures.

Updates #12711.

Change-Id: I5ee5879e25013ba1e6eda93ff280915f25ab5d55
Reviewed-on: https://go-review.googlesource.com/c/go/+/165898


Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>