Improve efficiency of fetching next opcode

[pliniker]

See interpreter/src/bytecode.rs:293 - multiple dereferences required to fetch a single opcode.

Ideal state: an instruction pointer that is a pointer to the next opcode.

Realistically: that might not be sensible, given that we probably want to retain bounds checking; what about extending Array with a RefCell-style ArrayOpcode slice borrow? That's as close to bare pointer in safe Rust as we can get.

Files:

• interpreter/src/bytecode.rs:293
• booksrc/chapter-interp-bytecode.md