factor out negative index handling to separate function, posrelat (to
match C Lua). When converting numbers to strings, see if the double value
can be represented exactly as a long, and if so format the string as
an integer. With these changes the standard test case "strings.lua" runs
to line 104 (over half way through!), where it uses string.format.
(1) Fix string.char and string.byte (argument indexing was off by one)
(2) Implement string.find
(3) Change calls to charAt, which does array bounds checking, in the
pattern matcher with calls to luaByte, which does not.
(4) Replace use of recursion with while/continue in pattern matcher.
