Frozen Functions Auto Opaque

[DoeringChristian]

This PR adds the following improvements to the frozen functions feature:

• An auto_opaque flag is introduced to the dr.freeze macro. If this flag is set (by default), the arguments of a frozen function are analyzed and if compatible only changing literal variables are made opaque. This significantly improves performance and memory usage of frozen functions.
• The opaque_width feature from drjit-core is used in the dr.mean function to determine the number of elements. This enables us to call dr.mean() inside a frozen function while keeping using the correct width when the input width to frozen functions changes.
• Improvements to logging, including a function to print "paths" to nodes in the PyTree if literals changed or when errors during traversal occurred.