Reduce fmt usage in String() methods

[case-fastly]

This addresses #199.

Also found a small bug in there:

When nearest3 rounds to a whole number (fract == 0), the function falls into the integer branch. But that branch formats int32(f) — the original unrounded input — not int32(nearest3).

Here's an example: https://go.dev/play/p/OahudI8zyfH

[dgryski]

The nearest3 code is a direct translation of the Rust: https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=878c4313bfa509636e7d46941e99c987

We should fix that too.

[dgryski]

LGTM. Only comment would be if we wanted to call .String() on a few of types instead of using string(x) so we don't break that encapsulation?

[dgryski]

LGTM