Goto program and SSA validation

[peterschrammel]

Introduce a --validate option to check goto programs and SSA for consistency.

• Replace cout/cerr in solvers #1875 (comment)
  (to be refined)

[tautschnig]

In #918 I made some first attempts in this direction, which immediately raised problems in the internal data structures.

[martin-cs]

Sounds like a great idea!

[tautschnig]

Additional suggestion: also check the symbol table for consistency, for example that every symbol has a non-empty mode field.

[tautschnig]

When working on this reverting (parts of) #2110 should be considered, and the same is true for #2046.

[tautschnig]

Closing this in favour of #3152. We have since made a lot of progress in this direction, and #3152 has pointers to various in-progress PRs.