replace datetime datatype with uint64

this remove datetime packets, adds a new general-purpose type U64 and
adjusts the state machine to work with unix timestamps instead of
subsets of struct tm.
extract statemachine stack from core code

this is mainly for the STM platform, which has rather small stacks.
devices will have a static stack, libhexabus machines have a
per-instance stack.
oops. reverse incorrect u32 order in u64 endiannes conversions
add a localtime endpoint that only the state machine can write

datetime packets were a really bad substitute for this.
remove timestamp packets

they were only useful for the old state machine, and even then they were
only a workaround.
remove obsolete statemachine ops enum