Adds expr functions for KV, KVV, Vars manipulation

- Adds/Refactors methods for Set, Merge, Filter, Delete
- Renames expr/Vars.Merge to MustMerge, updates its usage
- Appends kvFunctions to parser in pkg/expr
- Update/Fixes tests and example testable

pkg/expr/func_json.go

@@ -0,0 +1,18 @@
+package expr
+
+import (
+	"encoding/json"
+	"github.com/PaesslerAG/gval"
+)
+
+func JsonFunctions() []gval.Language {
+	return []gval.Language{
+		// TODO: Json decoding, parsing, stringify, JQ, JSONPath
+		gval.Function("toJSON", toJSON),
+	}
+}
+
+func toJSON(f interface{}) string {
+	b, _ := json.Marshal(f)
+	return string(b)
+}