Tracking Issue for Halide 16 release

[steven-johnson]

LLVM 16.0.0 has been released, so it's time to consider a release for Halide 16 at some point in the not-too-distant future. This is a tracking issue to gather up comments as to what in-progress code should be included in the release, and what should be deferred to Halide 17.)

[steven-johnson]

Possible things on the bubble:

• Xtensa backend
• Vulkan backend
• 'pip install' support
• StmtToViz suppot
• Deprecate the stmt_html output (to be subsumed by StmtToViz)