Allow retrieval of the program hash as an Opcode

## Problem

Upgrade procedures for applications may require comparing the hash of some program to some known and approved (signed) hash, then allowing the upgrade.

In order for this to work, the program must be <=4k since the stack does not allow a byte string larger than 4k.

## Solution

Provide an opcode that puts the hash of the program on the stack rather than forcing the hash to be done in logic.

## Urgency

Some projects need this already.