I think it would be useful to have a function like sqlite3_function_needed
called from the parser (or whatever module) to give the application an
opportunity to register the needed function seen in a SQL statement.
The idea came up from the sqlite3_collation_needed function which registers
a callback when a not already registered collation is seen/needed.
Having this new function, only the used funtions so far would be registered
(like collations) thus saving time and memory.
Sqlite internals (func.c) could also benefit from this api. This would speed
up the sqlite3_open call (which registers all the built in functions.)
