reorganized ZSTD_resetCCtx_internal()

reorganized ZSTD_resetCCtx_internal()

clearer separation between variables and buffers
clearer buffers category
kept static buffers at the beginning, favoring cache locality
(it will be easier to add FSE tables there later)

This break a few assumptions that hashTable was always at the beginning.
This is fixed.
And remaining assumptions (namely that tables stand next to each other in memory)
are now tested with assert.