Adds instr_noalloc_t, a wrapper around instr_t which contains space for all possible operands and a temporary encoding buffer. Adds support in the core instruction routines for using these internal fields and never using external heap, enabling this structure to be used for decoding in a signal handler.
The encoding buffer, due to reachability issues, cannot be re-relativized, making this structure inefficient for encoding. It is targeted toward decoding only.
Adds a sanity test.
Fixes #4180 (closed)