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.
uses plain Java arrays directly to keep heap allocation to a minimum.
Includes some unit tests that seem to indicate the basic operations are
correct. However, the following things are not implemented:
* Shrinking the capacity when elements are removed
* Optimal storage of each element in array vs. hash when entries are
added out of order. A junit test case is there for this.
