CBMC: Remove union/struct workaround (CBMC issue #8813)

[mkannwischer]

#818 introduced MLK_UNION_OR_STRUCT (should be MLD_UNION_OR_STRUCT) as a workaround for diffblue/cbmc#8813. It uses a struct by default (which works fine with CBMC), but switches to a union when MLD_CONFIG_REDUCE_RAM is set.
Once the issue is fixed in CBMC, we should remove the workaround, i.e.,

• Remove MLK_UNION_OR_STRUCT
• Use union everywhere
• Adjust the stack savings in the documentation of the config file.