Remove usage of assert.invariant

This reduces dependencies when building for the browser target, since
the assert module is a Node.js dependency. Simply replacing them with
an `if` statement is sufficient.

src/serialize.ts

@@ -1,4 +1,3 @@
-import invariant from 'assert'
 import { isInteger, getByteLength } from './helpers'
 
 function getClassNamespace(item: any, scope: Record<string, any>) {
@@ -59,7 +58,9 @@ export default function serialize(
   const constructorName = getClassNamespace(item, scope)
   if (typeof item.serialize === 'function') {
     const serialized = item.serialize()
-    invariant(typeof serialized === 'string', `${item.constructor.name}.serialize should return a string`)
+    if (typeof serialized !== 'string') {
+      throw new Error(`${item.constructor.name}.serialize should return a string`);
+    }
     return `C:${constructorName.length}:"${constructorName}":${getByteLength(serialized, options)}:{${serialized}}`
   }
   return `O:${constructorName.length}:"${constructorName}":${serializeObject(item, scope)}`

src/unserialize.ts

@@ -1,4 +1,3 @@
-import invariant from 'assert'
 // eslint-disable-next-line import/no-cycle
 import Parser from './parser'
 import { isInteger, getClass, getIncompleteClass, __PHP_Incomplete_Class } from './helpers'
@@ -11,7 +10,9 @@ export type Options = {
 function getClassReference(className: string, scope: Record<string, any>, strict: boolean): any {
   let container: any
   const classReference = scope[className]
-  invariant(classReference || !strict, `Class ${className} not found in given scope`)
+  if (!classReference && strict) {
+    throw new Error(`Class ${className} not found in given scope`);
+  }
   if (classReference) {
     // @ts-ignore
     container = new (getClass(classReference.prototype))()
@@ -93,7 +94,9 @@ function unserializeItem(parser: Parser, scope: Record<string, any>, options: Op
     const payload = parser.getByLength('{', '}', length => parser.readAhead(length))
     const result = getClassReference(name, scope, options.strict)
     if (!(result instanceof __PHP_Incomplete_Class)) {
-      invariant(result.unserialize, `unserialize not found on class when processing '${name}'`)
+      if (!result.unserialize) {
+        throw new Error(`unserialize not found on class when processing '${name}'`);
+      }
       result.unserialize(payload)
     }
     return result