Speedup encoder

[cristaloleg]

Thanks @quasilyte for the hint to try without bytes.Buffer.

goos: darwin
goarch: arm64
pkg: github.com/cristaloleg/benches/bencode
                        │ bench_old.txt │              bench.txt              │
                        │    sec/op     │   sec/op     vs base                │
_cristalhq_Marshal-10       597.5n ± 0%   534.6n ± 0%  -10.52% (p=0.000 n=10)
_cristalhq_MarshalTo-10     431.6n ± 0%   382.9n ± 0%  -11.26% (p=0.000 n=10)

                        │ bench_old.txt  │               bench.txt               │
                        │      B/op      │     B/op      vs base                 │
_cristalhq_Marshal-10      1024.0 ± 0%       976.0 ± 0%  -4.69% (p=0.000 n=10)
_cristalhq_MarshalTo-10     0.000 ± 0%       0.000 ± 0%       ~ (p=1.000 n=10) 

                        │ bench_old.txt │              bench.txt               │
                        │   allocs/op   │ allocs/op   vs base                  │
_cristalhq_Marshal-10      4.000 ± 0%     3.000 ± 0%  -25.00% (p=0.000 n=10)
_cristalhq_MarshalTo-10    0.000 ± 0%     0.000 ± 0%        ~ (p=1.000 n=10)