Optimize eager loads map key generation

[nicowolf91]

Following up on #1473, I improved the map key generation logic.

Primitive values, strings and byte slices in particular, no longer go through fmt.Sprintf, which reduces overhead. Numeric values are now handled by the strconv package, which is faster and produces fewer allocations.

SQL driver.Valuer types are unwrapped directly inside generateMapKey, which keeps the generated eager-loading code more readable and concise.

generateMapKey handles the typical data types used as database keys efficiently, with a best-effort fmt.Sprintf fallback for unrecognized types.

Let me know, if anything is missing!