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
```

generate/generate_test.go

@@ -124,7 +124,7 @@ func TestGenerate(t *testing.T) {
 	}
 }
 
-func defaultConfig(t *testing.T) *Config {
+func getDefaultConfig(t *testing.T) *Config {
 	// Parse the config that `genqlient --init` generates, to make sure that
 	// works.
 	var config Config
@@ -151,7 +151,7 @@ func TestGenerateWithConfig(t *testing.T) {
 		baseDir string  // relative to dataDir
 		config  *Config // omits Schema and Operations, set below.
 	}{
-		{"DefaultConfig", "", defaultConfig(t)},
+		{"DefaultConfig", "", getDefaultConfig(t)},
 		{"Subpackage", "", &Config{
 			Generated: "mypkg/myfile.go",
 		}},