perf(fmt): reduce memory usage and improve performance

[dsherret]

Reduces memory usage of the fmt command and fixes a memory leak.

Also slightly improves performance.

Before:

deno_std % hyperfine --prepare 'rm /Users/david/Library/Caches/deno/fmt_incremental_cache_v1' --warmup 5 '../deno/target/release/deno fmt'    
Benchmark 1: ../deno/target/release/deno fmt
  Time (mean ± σ):     155.4 ms ±  30.5 ms    [User: 1093.3 ms, System: 119.7 ms]
  Range (min … max):   132.8 ms … 248.8 ms    21 runs

After:

deno_std % hyperfine --prepare 'rm /Users/david/Library/Caches/deno/fmt_incremental_cache_v1' --warmup 5 '../deno/target/release/deno_new fmt'
Benchmark 1: ../deno/target/release/deno_new fmt
  Time (mean ± σ):     139.6 ms ±  12.2 ms    [User: 1099.0 ms, System: 112.1 ms]
  Range (min … max):   131.0 ms … 188.1 ms    20 runs