[Feature Request] A way to combine several identical queries instead of looping

I understand that this isn't production ready yet nor is it complete but I'd like to request a feature if you guys are taking those requests.

I have a small app that hits a graphql endpoint and returns ~50-60 product handles. I then need to individually query data about those products through their handle. The query for each individual product looks like this:

```
productByHandle(handle:$handle) {
variants(first: 35) {
edges {
node {
id
price
}
}
}
}
```

If I were to loop through each one and run the query individually I would hit a rate limit unless I specified something like `time.Sleep(500 * time.Millisecond)` after each query. But that adds up and takes time.

So what I ended up doing was writing a function that combines all the queries into named/aliased queries like this:

```
query {
product1: productByHandle(handle:"ABC") {
variants(first: 35) {
edges {
node {
id
price
}
}
}
}
product2: productByHandle(handle:"ZYX") {
variants(first: 35) {
edges {
node {
id
price
}
}
}
}
product3: productByHandle(handle:"XYZ") {
variants(first: 35) {
edges {
node {
id
price
}
}
}
}
product4: productByHandle(handle:"CBA") {
variants(first: 35) {
edges {
node {
id
price
}
}
}
}
...
...
...
}
```

which allows me to send it as one query instead of 50-60 individual ones.

So essentially what I am proposing is a method for doing essentially the same with genqlient; to be able to combine several identical queries into one large one. Thoughts?

No file or test is named in the issue. Start by tracing genqlient's query-generation and request entry points, then compare them with the aliased GraphQL example; done means repeated identical queries can be sent as one type-safe request with results mapped back to each product.

Written by the indexing model from the issue text.