Let HAVM have a more regular language.

	* src/Scan.hs (scanTokKeyword): Extend to support arithmetics in
	letters.
	Extend to handle symbols for arithmetics and comparisons.
	(isIndent, isIndentNum): Adjust to handle symbolic operators.
	(charToOp, scan): No longer treat (+) operators specifically.
	* src/Main.hs (evalM): The main function is main, not Main.
	* src/Ir.hs: Adjust the way to show the operators.
	* doc/havm.texi, doc/builtins.texi: Update to cover all the recent
	additions and changes.