Type-checked entryByIndex and readEntryOrNull

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Hi ClassFile API list,
I call for addition of a type-checked entryByIndex in ConstantPool, with a
signature:
<T extends PoolEntry> T entryByIndex(int index, Class<T> cls)
and a type-checked readEntryOrNull in ClassReader, with a signature:
<T extends PoolEntry> T readEntryOrNull(int offset, Class<T> cls)
Which will throw ConstantPoolException if the entry is of a mismatched
type, much like the type-checked readClassEntry in ClassBuilder.

A search for existing generic ConstantPool::entryByIndex and
ClassReader::readEntryOrNull in JDK reveals that most of their usages
within the jdk.internal.classfile.impl and its subpackages involve a direct
cast right after retrieving the result. These casts are susceptible to
malformed classfiles putting entries of wrong type, such as a Utf8 at the
cursor of superclass entry, throwing ClassCastException, which is out of
spec with the Classfile API.

I recommend adding these 2 methods for user convenience, and migrating all
existing entryByIndex/readEntryOrNull with casts to these 2 new methods, to
enhance the robustness of the ClassFile API. (On a side note, we can
promote ClassReader::utf8EntryByIndex to ConstantPool too)

Please feel free to comment or critique this proposal.

Chen Liang
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