add a way to get the aggregate context with createAggregateFunction

The old way of working was that we always passed the user context as
first argument to both `step` and `finalize` functions and the caller
had no way of getting the aggregate context from
SQLite (http://www3.sqlite.org/c3ref/aggregate_context.html).

Now both `step` and `finalize` functions must have a first argument of type `FunctionContext`:

    fn step(fctx: FunctionContext, input: u32) void {
	var ctx = fctx.aggregateContext(*u32) orelse return;
        ctx.* += input;
    }
    fn finalize(ctx: *u32) u32 {
	var ctx = fctx.aggregateContext(*u32) orelse return 0;
        return ctx.sum;
    }

Fixes #89

0 files changed, 0 insertions, 0 deletions