Convert ClassDesc and MethodTypeDesc to be stored in static final fields

The constant API are intended to be constable; for the Classfile API, in particular, the constant descriptors should be cached to avoid repeated validations, and MethodTypeDesc caches descriptor string as well. We should migrate their factory usages on call sites to reusing static final variables, starting from the JDK usages and the JMH benchmarks.

Related: https://mail.openjdk.org/pipermail/classfile-api-dev/2023-June/000378.html