The current implementation uses a max of 28 bits when decoding an
ObjectIdentifier.  This change makes it so that an int64 is used to
accumulate up to 35 bits.  If the resulting data would not overflow
an int32, it is used as an int.  Thus up to 31 bits may be used to
represent each subidentifier of an ObjectIdentifier.

Fixes #19933

Change-Id: I95d74b64b24cdb1339ff13421055bce61c80243c
Reviewed-on: https://go-review.googlesource.com/40436Reviewed-by: Adam Langley <agl@golang.org>
Run-TryBot: Adam Langley <agl@golang.org>