feat bundle: concolic generation, adapting latest TF, and more

[ganler]

1. concolic generation;
2. adapt tensorflow variable naming convention to use v[n]_[m] instead of v[n].[m];
3. a simplified memory limiter by constraining tensor element sizes;
4. improve gir visualization and text representation;
5. fix bugs in prior op spec;
6. for symbolic dims, be conservative by asserting s > 0; while for concrete dims, be relaxed by asserting s >= 0;