Reconstruct source code for FACTOR.

This is based on a disassembly of the binary AI:SYS1;TS FACTOR; the
binary and its help file .INFO.;FACTOR ORDER are both dated 1977-09-27.
MC had an identical copy with a later date.

This assembles into a binary that's identical to the original, except
that the original version has all its symbols marked as global, and no
assembly info.

Fixes #96.

doc/_info_/factor.order

@@ -0,0 +1,33 @@
+
+FACTOR IS USED FOR FACTORING INTEGERS, AND ALSO FOR
+CALCULATIONS WITH INTEGERS < 2^70 = 10^21
+
+COMMANDS TAKE PRECEDING NUMERIC ARGUMENTS
+EXAMPLE: 1+F ADDS 1 TO THE CURRENT VALUE, AND FACTORS IT
+
+‰RUBOUT IGNORES ITS ARGUMENT
+Q	FOLLOWED BY COMMAND, DESCRIBES COMMAND
+?	TYPES THIS FILE
+C	LISTS COMMANDS
+ 	SPACE STORES ARG IN CURRENT VALUE
+=	IGNORES ARG, TYPES CURRENT VALUE
+R	SETS INPUT RADIX TO ARG (MOD 2^35)
+D	SETS INPUT RADIX TO DECIMAL (IGNORES ARG)
+N	MOVES CURRENT VALUE INTO ARG SPACE; THUS N3 APPENDS A 3
+	TO THE CURRENT VALUE
+F	FACTORS ARG IF NOT ZERO; STORES INTO CURRENT VALUE
+	IF ARG IS 0, OR NO ARG, FACTORS CURRENT VALUE
+L	MOVES LARGEST FACTOR OF LAST NUMBER FACTORED
+	INTO CURRENT VALUE
++	ADDS ARG TO CURRENT VALUE
+;	ADDS ARG TO CURRENT VALUE
+-	SUBTRACTS ARG FROM CURRENT VALUE
+*	MULTIPLIES CURRENT VALUE BY ARG
+:	MULTIPLIES CURRENT VALUE BY ARG
+/	DIVIDES CURRENT VALUE BY LOW WORD OF ARG
+^	UPARROW TAKES LOW WORD OF CURRENT VALUE TO ARG POWER
+0-9	DIGITS
+
+OTHER CHARACTERS EVOKE ? AND ARE IGNORED; ANY NUMBER
+BEING ACCUMULATED IS NOT DISTURBED.
+IF PROGRAM IS RESTARTED, THE CURRENT VALUE IS NOT RESET