utils.signExtend

Includes a negative check so user code does not have to determine
whether to call padOcta or padSignedOcta.
implement STO operation

Also, utils.effectiveAddress now requires a uint64.
implement STBU, STWU, STTU, STOU

These are simply aliases for STB, STW, STT, and STO.
implement STHT operation
implement STCO operation
add register validation preconditions

Correct LDA to sum values from registers rather than immediateas.
Introduce Sum$XU utility for resolving, casting, and summing a variable
number of registers.
factor out common operation of summing $Y and $Z
jsdoc

Improve documentation comments and add jsdoc script.
implement ADD operation
implement SUB operation

int64$Y$Z utility adapts a function expecting register to a function
expecting one register and 2 int64's.  Refactor ADD operation to use
this adapter as well.
implement DIV operation
implement ADDU operation
implement SUBU operation
convert between decimal and hexadecimal