This extends `BUCKET` function to accept a two-parameters-only
invocation: the first parameter remains as is, while the second is a
span. It can be a numeric (floating point) span, if the first argument
is numeric, or a date period or time duration, if the first argument is
a date.
Also, the function can now be invoked with the alias BIN.
Additionally, the function has been turned into a grouping-only function
and thus can only be used within a `STATS` command.
This renames the function AUTO_BUCKET to just BUCKET.
It also removes the experimental tagging of the function in the docs, making it generally available.
Bogdan Pintea