Caching MethodType's descriptor string improves lambda linkage performance

MethodType.toMethodDescriptorString() is frequently invoked when generating lambda classes from InnerClassLambdaMetafactory.
Caching resulting string into the field of MethodType gives +5% - +10% to lambda linkage performance.

Minor performance improvement: private method "checkPtype" was inlined and eliminated. "checkRtype" and "checkPtypes" were refactored for better perfomance in HotSpot interpreter (important for lambda linkage). overall result +1%.

Attached webrev contains fix: please review.