Remove use of packed structs.

Remove use of packed structs, now that the infrastructure for consistently
aligning types between 32-bit and 64-bit platforms is in place. This fixes
crashes on platforms which enforce alignment rules (because there are
places in the code where pointers to packed fields are used).

Also, it theoretically improves performance, though I haven't actually
performed any measurements.

ABI implications:

jack_latency_range_t and jack_position_t are no longer packed. The layouts
of these structs themselves are unmodified, though the layouts of any
structs which include these structs as members may be affected.