Update underlying number representation

These changes modify OPA to use json.Number to represent number values in
storage and the AST. Comparisons and numeric operations are handled by the
math/big package. Using json.Number avoids loss of precision when loading
integers greater than 2**53 and use of math/big avoids need for type
conversions in the built-in implementations. In the long run, the math/big
usage may be replaced with specific paths for smaller numbers.

Fixes #154