The following patchset refactors the AAC decoder step by step,
removing all large-scale templating and abstracting away typed
DSP functions from symbol parsing.

This series saves 110Kib of binary data from libavcodec.so
when compiled with Clang 18 with -O3, after stripping.

The patchset can also be viewed here:
https://github.com/cyanreg/FFmpeg/tree/aac_cleanup

Fate passes on each step, and I've been running this patchset for
a week with no issues.

This also prepares the decoder for USAC support.