Start on a bool type for gawk.

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+Fri Mar  5 13:42:44 IST 2021
+============================
+
+Design Notes for a Boolean Type in Gawk
+
+1. At first glance, two new variables named TRUE and FALSE would make sense
+to be predefined values of boolean type.  However, since it's likely that
+many awk programs already use them, instead we'll add two new entries
+to PROCINFO: variables PROCINFO["true"] and PROCINFO["false"] of type bool.
+
+Assigning from them copies the bool type, thus they are "factories" of bool
+variables.
+
+2. They have numeric values 1 and 0 respectively, and string values
+"TRUE" and "FALSE".  Thus they differ from other variables where a
+"false" value must be zero and null.
+
+This implies all of the following:
+
+	print(PROCINFO["true"]) --> "TRUE"
+	print(PROCINFO["false"]) --> "FALSE"
+	Same for %s in printf
+	Same for bool_var ""
+	printf %d gives 0/1
+
+3. Function  bool(val)  converts val to bool, returning Boolean TRUE or FALSE.
+
+4. typeof() returns "bool".
+
+5. Numeric operators treat booleans as numbers.  asort() sorts booleans before
+numbers, and false before true.
+
+6. These string function generate a runtime fatal error
+if given an argument / target of boolean type:
+
+	substr	match	index	gensub	gsub
+	sub	length	split	patsplit
+	tolower	toupper
+
+7. Updates to API needed for an additional type, and the table
+for requested vs. returns.
+
+8. The following extensions need revising:
+
+	- JSON extension
+	- dump / read array extensions
+	- what else?