Add a reset function for the TonalityAnalysisState struct
and call it on encoder reset.

Move the state struct above the clear line in OpusEncoder
so reset doesn't clobber reusable fields. Currently this
is only the arch field, which is moved to to top of the
struct so we can use the same memset-to-the-end pattern
as OpusEncoder.

Signed-off-by: Jean-Marc Valin <jmvalin@jmvalin.ca>