Add an abstraction for "uninitialized or"

[arthaud]

The current analysis is imprecise when merging branches where one side has uninitialized variables.

For instance:

#include <ikos/analyzer/intrinsic.h>

int main(int argc, char** argv) {
    int tab[10];

    if (argc > 2) {
        tab[0] = 2;
    }

    __ikos_print_values("tab[0]", tab[0]);

    return 0;
}

The output of the analyzer is:

test.c:10:5: __ikos_print_values("tab[0]", tab[0]):
	tab[0] is in [-2147483648, 2147483647]
	tab[0] may be uninitialized

We could add an abstraction that remembers whether a value is uninitialized or within a given range.

This should help with #136.