Implicit indicator for loop headings and AN/OR

[Jeroen T. Vermeulen]

A better solution for the AN/OR chaining of conditionals, as well as the semantics of those conditionals at the beginning of loops:

Create an implicit indicator.

An instruction line without opcode sets the implicit indicator (we can register None as an opcode).

An AN/OR line performs a logical and/or respectively between the implicit indicator and the line's own indicators.

We split a conditional loop header (DoW/DoU) into two instructions: the first is an opcodeless one and computes the implicit indicator, and the second is a loop header conditional only on the implicit indicator.

The end of a loop sets the implicit indicator to "true," and jumps to the loop header.