Support multibyte opcodes

## Problem

Since opcode values are limited to a single byte, related behaviors are expressed as unique opcodes. For example, these behaviors must be expressed as unique opcodes:
* Operations common BN256 (https://github.com/algorand/go-algorand/pull/4013) and BLS12-381 (https://github.com/algorand/go-algorand/issues/4036).
* Operations common to Boxes (e.g. `box_put`, `box_len`, `box_get` via https://github.com/algorand/go-algorand/pull/4212).

It's possible to imagine supporting multibyte opcode values to group common behaviors and limit usage of the top-level opcode space. With boxes, imagine `box ` like `box put`, `box len`, `box get`.

## Solution

The assembler should be augmented to allow OpSpecs to exist "beneath" a single opcode byte. This would effectively cause those opcodes to be assembled as a two-byte sequence.

## Urgency

The current encoding scheme is running out of opcode space.