Replace flatten implementation to reduce bundle size

package.json

@@ -1,7 +1,12 @@
 {
   "name": "@validatem/core",
   "description": "The last validation library you'll ever need.",
-  "keywords": ["validation", "input", "verification", "data"],
+  "keywords": [
+    "validation",
+    "input",
+    "verification",
+    "data"
+  ],
   "version": "0.3.0",
   "main": "index.js",
   "repository": "http://git.cryto.net/validatem/core.git",
@@ -18,10 +23,10 @@
     "@validatem/normalize-rules": "^0.1.0",
     "@validatem/required": "^0.1.0",
     "@validatem/validation-result": "^0.1.1",
-    "array.prototype.flat": "^1.2.3",
     "assure-array": "^1.0.0",
     "create-error": "^0.3.1",
     "default-value": "^1.0.0",
+    "flatten": "^1.0.3",
     "is-arguments": "^1.0.4"
   },
   "devDependencies": {

src/api/validate-arguments.js

@@ -1,7 +1,7 @@
 "use strict";
 
 const isArguments = require("is-arguments");
-const flat = require("array.prototype.flat");
+const flatten = require("flatten");
 
 const ValidationError = require("@validatem/error");
 const annotateErrors = require("@validatem/annotate-errors");
@@ -46,7 +46,7 @@ module.exports = createValidationMethod((args, argumentDefinitions) => {
 		});
 
 		let combinedErrors = results.map((result) => result.errors);
-		let flattenedErrors = flat(combinedErrors);
+		let flattenedErrors = flatten(combinedErrors); // TODO: Switch to `Array#flat` once Node 10.x goes EOL (April 2021)
 
 		let newValues = results.map((result) => result.newValue);
 

yarn.lock

@@ -604,6 +604,11 @@ flatted@^2.0.0:
   resolved "https://registry.yarnpkg.com/flatted/-/flatted-2.0.2.tgz#4575b21e2bcee7434aa9be662f4b7b5f9c2b5138"
   integrity sha512-r5wGx7YeOwNWNlCA0wQ86zKyDLMQr+/RB8xy74M4hTphfmjlijTSSXGuH8rnvKZnfT9i+75zmd8jcKdMR4O6jA==
 
+flatten@^1.0.3:
+  version "1.0.3"
+  resolved "https://registry.yarnpkg.com/flatten/-/flatten-1.0.3.tgz#c1283ac9f27b368abc1e36d1ff7b04501a30356b"
+  integrity sha512-dVsPA/UwQ8+2uoFe5GHtiBMu48dWLTdsuEd7CKGlZlD78r1TTWBvDuFaFGKCo/ZfEr95Uk56vZoX86OsHkUeIg==
+
 fs.realpath@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/fs.realpath/-/fs.realpath-1.0.0.tgz#1504ad2523158caa40db4a2787cb01411994ea4f"