Creates a `ROUND_TO` function that rounds it's input to one of the
provided values. Like so:
```
ROUND_TO(v, 0, 5000, 10000, 20000, 40000, 100000)
v | ROUND_TO
0 | 0
100 | 0
6000 | 5000
45001 | 40000
999999 | 100000
```
For some sequences of numbers you could do this with the `/` operator -
but for arbitrary sequences of numbers you needed `CASE` which is quite
slow. And hard to read!
Rewriting the example above would look like:
```
CASE (
v < 5000, 0,
v < 10000, 5000,
v < 20000, 10000,
v < 40000, 20000,
v < 100000, 40000,
100000
)
```
Even better, this is *fast*:
```
(operation) Mode Cnt Score Error Units
round_to_4_via_case avgt 7 138.124 ± 0.738 ns/op
round_to_4 avgt 7 0.805 ± 0.011 ns/op
round_to_3 avgt 7 0.739 ± 0.011 ns/op
round_to_2 avgt 7 0.651 ± 0.009 ns/op
date_trunc avgt 7 2.425 ± 0.018 ns/op
```
I've included a comparison to `DATE_TRUNC` above because we should be
able to rewrite `DATE_TRUNC` into `ROUND_TO` when we know the date range
of the index. This doesn't do it now, but it should be possible.
45bfaab448