Speed up lexer more by changing from rune to byte type
Speed up lexer more by changing from rune to byte type

benchmark                   old ns/op     new ns/op     delta
BenchmarkNumbers-8          4749          1923          -59.51%
BenchmarkKeywords-8         5689          2313          -59.34%
BenchmarkNames-8            4901          2186          -55.40%
BenchmarkStrings-8          5079          2282          -55.07%
BenchmarkRegex-8            4183          1966          -53.00%
BenchmarkProgram-8          2563          1483          -42.14%
BenchmarkChoiceTokens-8     2065          2127          +3.00%
BenchmarkSimpleTokens-8     902           918           +1.77%

benhoyt/goawk,benhoyt/goawk,benhoyt/goawk