Assignment typet strong typing and UB fix

[reuk]

The most important part of this patch is the added initialisers in symex_target_equation.h, which fix this warning:

symex_target_equation.h:167:9: runtime error: load of value 3432879264, which is not a valid value for type 'assignment_typet'

This error was observed in diffblue/platform#520, when cbmc was built using clang's undefined behaviour sanitizer.

Although only the addition on line 209 is strictly required, I've also added initializers for the other primitive data members, because objects should always be in a known state which maintains their invariants once the constructor has finished.

Finally, I changed assignment_typet into an enum class to improve its type-safety and avoid undesired conversions.

[tautschnig]

Is this genuinely a test-gen-support problem only?

[reuk]

Possibly not - closing this in favour of #872.