fix a bug in type hashconsing

[deyaaeldeen]

The index field in types was not being updated correctly. Before the fix, 0 was written to the index field, regardless of the current value of types_unique_index_counter, because Types-gen-index! has side effects and its result is not written to the variable.

Furthermore, I decreased some code bloat by taking advantage of uniform memory layout of structural types.

Finally, I fixed a memory leak when resizing the types hashconsing table.