20160706 - Low Cost Branching to Factoring Out Loop Exit Check

Threading to hide pipeline depth combined with an ISA which makes branching cheep is one goal. Specifically absolute branching with immediate destination in the opcode word (single word branch/call, no adder), and instructions which include a return flag (make returns free). Enables easy computed branches, both for jump tables, and loops. Can factor out loop check into hierarchical call tree,

Do4: Unroll work four times; Return;
Do16: Call Do4; Call Do4; Call Do4; Jump Do4;
Do64: Call Do16; Call Do16; Call Do16; Jump Do16;
... etc ...

Can use a computed branch to jump into the tree for other loop counts.