This is used by the flow analysis code to keep track of the type of local variables and variables. The flow code uses a BitVector to keep track of whether a variable has been assigned or not. This is easy for fundamental types (int, char etc.) or reference types since you can only assign the whole variable as such. For structs, we also need to keep track of all its fields. To do this, we allocate one bit for the struct itself (it's used if you assign/access the whole struct) followed by one bit for each of its fields. This class computes this `layout' for each type.
