Only convert Scientific to Integer for nonnegative exponents

[abooij]

We use attoparsec's scientific number parsing code, which gives us a Scientific value. We like to work with Integers when we can, and so we sometimes convert this scientific number to an integer by simply taking its floor. But simply checking if the input text has a period symbol . in it does not suffice to check for correctness of this conversion, as numbers with a negative exponent may not be integral. This fixes that to only store numbers as integers if they have been supplied with a nonnegative exponent.

hasura/graphql-engine#4733 will be fixed by this (after we update the version of graphql-parser-hs used there).

[abooij]

I should add that another option is to check whether the input expression is in scientific notation, and decide based on that, rather than depending on the actual value. In the version implemented in this PR, an input value of, say, 8E3 is saved as the Integer value 8000. Under the alternative scheme, 8E3 would be stored as a Scientific value. Perhaps this is more true to "scientific notation" interpretation of the Scientific type.

I leave it up to the reviewer to make a decision on this.