fmt: simplfy and reduce size of integer formatting

[BurntSushi]

This more completely unifies unsigned and signed integer
formatting. It also removes an unused option (to force the
printing of the sign) and an unused field.

Interestingly, these changes result in quite a substantial
(relatively speaking) decrease in LLVM lines. In my Biff
benchmark, prior to this PR, we were getting 753,181 LLVM
lines. But with this PR, we get 745,928 LLVM lines.

For compile times on my Biff benchmark (which is just doing
a cargo build and a cargo build --release after a
cargo clean for each), it seems like there's basically
no change. With maybe the release build being a hair faster.

Before this PR:

Benchmark 1: cargo b
  Time (mean ± σ):      4.659 s ±  0.023 s    [User: 15.631 s, System: 2.070 s]
  Range (min … max):    4.618 s …  4.694 s    10 runs

Benchmark 2: cargo b -r
  Time (mean ± σ):      7.770 s ±  0.076 s    [User: 65.860 s, System: 2.556 s]
  Range (min … max):    7.656 s …  7.886 s    10 runs

After this PR:

Benchmark 1: cargo b
  Time (mean ± σ):      4.654 s ±  0.033 s    [User: 15.521 s, System: 2.079 s]
  Range (min … max):    4.606 s …  4.717 s    10 runs

Benchmark 2: cargo b -r
  Time (mean ± σ):      7.651 s ±  0.053 s    [User: 65.295 s, System: 2.560 s]
  Range (min … max):    7.575 s …  7.711 s    10 runs