Fix #82 and #148

Add --custom-storage-layout option that allows for custom storage layouts.

The idea is to use a simple injective function to implement hash semantics that are collision-free (but not secure). This function simply left-shifts the input by 257 bits, where 256 bits ensure enough logical domain space, and the additional 1 bit is used for disambiguation (e.g., differentiating between mapping (uint => uint) and uint[][]). Note that the output of this function is simple enough for the solver to easily reason about the non-aliasing between different storage variables.