NestHost/NestMembers were carried as Attribute::Raw with constant-pool
indices baked in. During writing the pool is rebuilt in a different order,
so those indices pointed at the wrong entries and the JVM rejected the
class: ClassFormatError: Nest member/host class_info_index N has bad
constant type.
Model them explicitly in the IR, convert on parse and write (resolving
class names through the new pool), and remap their class names during
rename. Add a NestMembers round-trip regression test. Verified on a real
jar: all Nest ClassFormatErrors gone.
