Embed data files in the binary (#180)

Now that we're on Go 1.16+ we can do this easily! The main advantage is
it means users can build a genqlient binary and use that portably (or we
could distribute one, or whatever). Plus the code is marginally simpler;
the `embed` API is really quite nice.

Fixes #9.

Test plan:
```
make check
go build .
rm -rf generate               # pretend we have no checkout
./genqlient ./internal/integration/genqlient.yaml
./genqlient --init            # fails after generating a default config
```

docs/CHANGELOG.md

@@ -26,6 +26,8 @@ When releasing a new version:
 
 ### New features:
 
+- genqlient can now run as a portable binary (i.e. without a local checkout of the repository or `go run`).
+
 ### Bug fixes:
 
 ## v0.4.0