Strip o_ prefix from ObjectData::o_to{Boolean,Int64,Array}[Impl]

Summary: It was only there for historical reasons. Also remove a bogus comment about the
*Impl methods being overridable.

I'm planning on splitting ObjectData::toArray(bool) out into two methods
toArray() and toPubArray(), but that's out of the scope of this diff.

Reviewed By: @jdelong

Differential Revision: D1707863

Signature: t1:1707863:1417198230:f32d1ba27a3f4c71ead3569f38a9ed9b373fb96f