Fix Bugs, Performance Improvement

* Metadata: [BUG FIX] Corrected the mistake that handles for the token ELEMENT_TYPE_VAR and ELEMENT_TYPE_MVAR. They are not assigned the token of TypeSpec while they are only existing in the signature, but the token is assigned when they are used in the method body.
* Metadata: Improved the finding method performance for custom attributes.
* StrongNaming: [BUG FIX] Added the judgment condition for the assembly that doesn't have strong name.