fix(graphql-server): integer coercion in variables

[gligneul]

Without this patch, the integer variables are always interpreted as BigInts.

Fixes cartesi/rollups-node#81

[tuler]

Is it necessary to check the value to decide how to parse?

[gligneul]

Is it necessary to check the value to decide how to parse?

The other options are:

• Always converting to int32 integers since we don't have int64 variables.
• Don't convert and change the variables to int64. I don't like this option because it will change the schema.

[tuler]

Don't you know the type of the variable from its definition?

[gligneul]

@tuler, when deserializing from a JSON, the library doesn't know the type of the value it expects. It reads all integer values as int64, so it needs to change the type given the value. More info: https://docs.rs/juniper/latest/juniper/trait.ScalarValue.html