Scope of SHARING macro in util/irep.h is too great

[reuk]

Whether or not irept uses sharing is an implementation detail. Having the SHARING switch in the header means that if I want to disable sharing, I have to rebuild all of CBMC instead of just the irep implementation. Also, this means that SHARING gets defined in every file which includes irep.h, which (as it is not a particularly long name) is likely to collide with another macro or enum identifier at some point.