py: Reduce size of mp_code_state_t structure.
Instead of caching data that is constant (code_info, const_table and n_state), store just a pointer to the underlying function object from which this data can be derived. This helps reduce stack usage for the case when the mp_code_state_t structure is stored on the stack, as well as heap usage when it's stored on the heap. The downside is that the VM becomes a little more complex because it now needs to derive the data from the underlying function object. But this doesn't impact the performance by much (if at all) because most of the decoding of data is done outside the main opcode loop. Measurements using pystone show that little to no performance is lost. This patch also fixes a nasty bug whereby the bytecode can be reclaimed by the GC during execution. With this patch there is always a pointer to the function object held by the VM during execution, since it's stored in the mp_code_state_t structure.