Update bindings to be compatible with Nix 2.30, and package with Nix

.gitignore

@@ -1,2 +1,5 @@
 .devenv
 .direnv
+.idea/
+
+result

err.go

@@ -1,17 +1,18 @@
 package gonix
 
-// #cgo pkg-config: nix-expr-c
+// #cgo pkg-config: nix-expr-c nix-main-c
 // #include <stdlib.h>
 // #include <nix_api_util.h>
 // #include <nix_api_expr.h>
 // #include <nix_api_value.h>
+// #include <nix_api_main.h>
 import "C"
 import (
 	"errors"
 	"fmt"
 )
 
-func nixError(err C.int, ctx *Context) error {
+func nixError(err C.nix_err, ctx *Context) error {
 	switch err {
 	case C.NIX_OK:
 		return nil

external.go

@@ -61,7 +61,7 @@ type ExternalValueProvider interface {
 	CoerceToString(addContext func(string) error, copyMore, copyToStore bool) (string, error)
 
 	// Equal is called for a comparison of two external values. If `other` is not an external
-	// value, Equal isn't called and instead `false` is always returned.
+	// cvalue, Equal isn't called and instead `false` is always returned.
 	Equal(other ExternalValueProvider) bool
 
 	// PrintValueAsJSON is called when the value is converted to JSON. The result must
@@ -72,7 +72,7 @@ type ExternalValueProvider interface {
 	// PrintValueAsXML(strict, location, copyToStore bool) string
 }
 
-// ExternalValue is a wrapper around a go value passed into nix.
+// ExternalValue is a wrapper around a go cvalue passed into nix.
 type ExternalValue struct {
 	cev *C.ExternalValue
 }

external_test.go

@@ -28,7 +28,7 @@ func (v *simpleExternal) Equal(other gonix.ExternalValueProvider) bool {
 }
 
 func (v *simpleExternal) PrintValueAsJSON(strict, copyToStore bool) string {
-	return `{ "value" : "simpleexternal" }`
+	return `{ "cvalue" : "simpleexternal" }`
 }
 
 func ExampleState_NewExternalValue() {
@@ -91,5 +91,5 @@ func ExampleExternalValueProvider_PrintValueAsJSON() {
 	}
 
 	fmt.Println(res)
-	// Output: {"value":"simpleexternal"}
+	// Output: {"cvalue":"simpleexternal"}
 }