Questions to think about:

1. double field is added to the types module in the tarantool_graphql module.
2. Float is 64-bt in GraphQL, AFAIR. We need to convert avro-schema double into GraphQL Float. Maybe we need also create Double type that will be full alias of the Float (and can be used instead in any place). Possible problem: use a Double variable for a Float argument and vice versa.
3. You are use node.kind == 'double', but core/parser.lua does not emit such nodes (it emit float ones).
4. No tests.

Closed in favor of PR #128. Roman, thanks for the effort!