feat(parser): export parser.Children (#327)

* feat(parser): export parser.Children

As parser was moved to internal/parser, it is not possible to import it into
external projects anymore.

However, parser.Children() is handy to use in static analysis tools, so it is
worth exporting. This does that by adding a stub package
`github.com/google/go-jsonnet/parser` that wraps the internal function.

Signed-off-by: sh0rez <me@shorez.de>

toolutils/BUILD.bazel

@@ -0,0 +1,12 @@
+load("@io_bazel_rules_go//go:def.bzl", "go_library")
+
+go_library(
+    name = "go_default_library",
+    srcs = ["ast.go"],
+    importpath = "github.com/google/go-jsonnet/toolutils",
+    visibility = ["//visibility:public"],
+    deps = [
+        "//ast:go_default_library",
+        "//internal/parser:go_default_library",
+    ],
+)

toolutils/ast.go

@@ -0,0 +1,12 @@
+// Package toolutils includes several utilities handy for use in code analysis tools
+package toolutils
+
+import (
+	"github.com/google/go-jsonnet/ast"
+	"github.com/google/go-jsonnet/internal/parser"
+)
+
+// Children returns all children of a node. It supports ASTs before and after desugaring.
+func Children(node ast.Node) []ast.Node {
+	return parser.Children(node)
+}