Add getFieldInitializer to ExtendedRemapper

My rough plan for combining static scrambling and remapping is to split
Library::remap into three passes:

* Remove static methods, fields and initializers from the library,
  storing them in a temporary location.
* Pass all classes through ClassNodeRemapper, as we do now.
* Add static methods, fields and initializers to their new classes,
  remapping as we do so.

This ensures a ClassNode is never in a state where it has a mixture of
remapped and non-remapped fields, methods or instructions. This is
important to ensure no conflicts can occur when we use the refactored
names from the NameMap, rather than the auto-generated names.

It means TypedRemapper needs the ability to provide the instructions
that make up a field's initializer, such that Library::remap can move
these instructions to a different InsnList. The new getFieldInitializer
method and FieldInitializer type support this.

Signed-off-by: Graham <gpe@openrs2.dev>

asm/src/main/java/dev/openrs2/asm/classpath/ExtendedRemapper.kt

@@ -3,6 +3,10 @@ package dev.openrs2.asm.classpath
 import org.objectweb.asm.commons.Remapper
 
 abstract class ExtendedRemapper : Remapper() {
+    open fun getFieldInitializer(owner: String, name: String, descriptor: String): FieldInitializer? {
+        return null
+    }
+
     open fun mapFieldOwner(owner: String, name: String, descriptor: String): String {
         return mapType(owner)
     }

asm/src/main/java/dev/openrs2/asm/classpath/FieldInitializer.kt

@@ -0,0 +1,14 @@
+package dev.openrs2.asm.classpath
+
+import dev.openrs2.asm.MemberRef
+import org.objectweb.asm.Opcodes
+import org.objectweb.asm.tree.AbstractInsnNode
+import org.objectweb.asm.tree.FieldInsnNode
+
+class FieldInitializer(val instructions: List<AbstractInsnNode>, val version: Int, val maxStack: Int) {
+    val dependencies = instructions.asSequence()
+        .filterIsInstance<FieldInsnNode>()
+        .filter { it.opcode == Opcodes.GETSTATIC }
+        .map(::MemberRef)
+        .toSet()
+}