yang-parser-impl: activation of deviation statement

As it turns out, based on https://mailarchive.ietf.org/arch/msg/netmod/FcseqgXgdNhOb3k_BgN1aa8By5M, the deviation statement needs to be explicitly activated and is not effective just by virtue of being present in the library.

Fix handling of deviations so that the caller can tell us which module has enabled deviations, effectively passing in the information implied by https://tools.ietf.org/html/rfc6020#section-5.6.4.

Note this requires API changes, which need to remain compatible with previous behavior.