discriminate: change way to configure the transformation
This commit enable the possibility to change discriminate behavior from Why3 source files. The 4 metas that configure the transformation: select_inst select_lsinst select_lskept select_kept can now be configured from source files (actually they could before, but their value was overriden by the drivers). The behavior in absence of annotation can be specified from drivers using the 4 new configuration metas: select_inst_default select_lsinst_default select_lskept_default select_kept_default They behave as their non-default counterparts, except they have lower precedence. This avoid the forementioned overriding problem.
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