fix createAggregateFunction

[vrischmann]

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.

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