gql-dedup: a graphql-tag improvement library which de-duplicates fragments

Well-known Apollo's graphql-tag
library hasn't been updated for a long time, and there has been several PRs got
stuck there.

The current library solves one common weakness of graphql-tag: it allows to
refer fragments multiple times and have them de-duplicated in the query.

Example

Imagine we have a common FragDate fragment which two other fragments,
FragCreated and FragUpdated, depend on. Then, with the default graphql-tag
library, using of both FragCreated and FragUpdated in the same query won't
be possible since FragDate would be included twice, and we'd get a server-side
error.

Here comes graphql-tag: it de-duplicates fragments each time they're used.

``ts
import gql from "gql-dedup";

const FragDate = gql
fragment FragDate on User {
day
month
year
}
;

const FragCreated = gql
fragment FragCreated on User {
created {
...FragDate
}
}
${FragDate}
;

const FragUpdated = gql
fragment FragUpdated on User {
updated {
...FragDate
}
}
${FragDate}
;

const Query = gql
query User {
viewer {
...FragCreated
...FragUpdated
}
}
${FragCreated}
${FragUpdated}
;
`

Result (notice that FragDate is included only once):

`graphql
query User {
viewer {
...FragCreated
...FragUpdated
}
}
fragment FragCreated on User {
created {
...FragDate
}
}
fragment FragDate on User {
day
month
year
}
fragment FragUpdated on User {
updated {
...FragDate
}
}
``