Partial Application

Implement partial(fn, ...presetArgs) that returns a new function with
some arguments pre-filled. When the returned function is called, it prepends
presetArgs to any new arguments before forwarding to fn.

solve(presets, calls) drives the implementation using a variadic-sum
function.

Example

const add = (a, b, c) => a + b + c;
const add10 = partial(add, 10);
add10(5, 5)  // 20  → add(10, 5, 5)