Smaller fixes

[rayman2000]

• Improved the error message for unsupported features
• Fixed a bug in list_mul: the values in the result were being calculated incorrectly
• Added support for list.insert: the insert index is complicated, if it is greater than the length, then we append at the end. If it is negative, we go backwards from the end of the list, if negative and greater than the length, we prepend.

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This change is 

[rayman2000]

• Changed the encoding of contains for lists and PSeqs. They now state that an element exists in the collection that object-equals the input.

[rayman2000]

• Added --strict-int mode, which replaces many instances of isinstance(x, int) with type(x) == int in the Viper encoding.