large repeat expressions don't adhere to the execution limit

[oli-obk]

and are really slow, since they use write_value for every single value.

I have some code where I write_value the first value and then just copy those bytes N-1 times over the remaining array fields. Better ideas are very welcome.

old speed:

test repeat        ... bench:     509,773 ns/iter (+/- 96,298)
test repeat_manual ... bench: 313,843,922 ns/iter (+/- 5,732,707)

new speed:

test repeat        ... bench:     366,520 ns/iter (+/- 69,298)
test repeat_manual ... bench: 300,833,908 ns/iter (+/- 8,937,121)

not really worth the improvements. I'll just slap on an execution limit thingy.