Database functions currently receive their arguments as an std::vector. This
is inflexible (for example, one cannot use small_vector to reduce allocations).
This series adapts the function signature to accept parameters using std::span.
Some changes in the keys interface are needed to support this. Lastly, one call
site is migrated to small_vector.
This is in support of changing selectors to use expressions.
Closes#13581
* github.com:scylladb/scylladb:
cql3: abstract_function_selector: use small_vector for argument buffer
db, cql3: functions: pass function parameters as a span instead of a vector
keys: change from_optional_exploded to accept a span instead of a vector
d828cfcb23