Add Method/Field iterator to ClassAccessor

Enables ranged based for loops on fields and methods.

For visiting both fields and methods, VisitFieldsAndMethods will
be faster because of not needing to decode the fields twice for
seeking purposes.

Added test.

Bug: 79758018
Bug: 77709234
Test: test-art-host-gtest

Change-Id: I593e23ccd138b87a27d8bab6927ff2b685c057f3