Split callback tables

[mgaudet]

/** 
 * 
 * This struct contains function pointers that are used by the JIT to callback. 
 *
 * The function pointers can be broken into two groups. The first group are
 * generated callbacks; These are callbacks emitted by insns.def, the
 * instruction definition file. 
 *
 * The second segment contains callbacks to VM functionality that may be 
 * required by JITs. 
 */
struct jit_callbacks_struct {
  // ...
};

This table should actually be broken into two pieces (to assist in making #30 easier to do safely).

1. Function pointers passed to the JIT for the purpose of code generation.
2. Function pointers passed to the JIT so the JIT can call them.

[mgaudet]

Closed by #98