refactor(core): adjust directory structure:

src/core/util/core.js

@@ -0,0 +1,56 @@
+/**
+ * Create a cached version of a pure function.
+ */
+function cached (fn) {
+  const cache = Object.create(null)
+  return function cachedFn (str) {
+    const hit = cache[str]
+    return hit || (cache[str] = fn(str))
+  }
+}
+
+/**
+ * Camelize a hyphen-delimited string.
+ */
+const camelizeRE = /-(\w)/g
+export const camelize = cached((str) => {
+  return str.replace(camelizeRE, (_, c) => c ? c.toUpperCase() : '')
+})
+
+/**
+ * Simple Object.assign polyfill
+ */
+export const merge = Object.assign || function (to) {
+  const hasOwn = Object.prototype.hasOwnProperty
+
+  for (let i = 1; i < arguments.length; i++) {
+    const from = Object(arguments[i])
+
+    for (const key in from) {
+      if (hasOwn.call(from, key)) {
+        to[key] = from[key]
+      }
+    }
+  }
+
+  return to
+}
+
+/**
+ * Check if value is primitive
+ */
+export function isPrimitive (value) {
+  return typeof value === 'string' || typeof value === 'number'
+}
+
+/**
+ * Perform no operation.
+ */
+export function noop () {}
+
+/**
+ * Check if value is function
+ */
+export function isFn (obj) {
+  return typeof obj === 'function'
+}