Don't use bytes.Buffer when not necessary

[pelletier]

When parsing strings, they can be referenced directly from the document when they don't contain escaped characters. This avoids paying to cost of allocating (and sometimes growing) the bytes buffer unnecessarily.

The next step after this will likely to use unsafe to also avoid the copy for the string when not needed (e.g when doing lookup of existing entries in maps, or struct fields).

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name                              old time/op    new time/op    delta
UnmarshalDataset/config-32          40.1ms ± 2%    31.5ms ± 2%  -21.44%  (p=0.000 n=9+10)
UnmarshalDataset/canada-32           111ms ± 2%     101ms ± 3%   -8.52%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32    42.4ms ± 3%    37.9ms ± 2%  -10.61%  (p=0.000 n=9+10)
UnmarshalDataset/twitter-32         17.0ms ± 3%    13.9ms ± 2%  -18.31%  (p=0.000 n=10+9)
UnmarshalDataset/code-32             133ms ± 5%     119ms ± 2%  -10.04%  (p=0.000 n=10+9)
UnmarshalDataset/example-32          288µs ± 4%     245µs ± 4%  -15.08%  (p=0.000 n=10+10)
UnmarshalSimple/struct-32           1.10µs ± 4%    0.87µs ± 1%  -21.10%  (p=0.000 n=10+9)
UnmarshalSimple/map-32              1.53µs ± 3%    1.26µs ± 3%  -17.84%  (p=0.000 n=9+10)
ReferenceFile/struct-32             66.6µs ± 5%    55.9µs ± 5%  -16.06%  (p=0.000 n=10+10)
ReferenceFile/map-32                 110µs ± 4%      92µs ± 4%  -16.67%  (p=0.000 n=10+10)
HugoFrontMatter-32                  25.3µs ± 6%    19.0µs ± 3%  -24.73%  (p=0.000 n=9+10)

name                              old speed      new speed      delta
UnmarshalDataset/config-32        26.2MB/s ± 2%  33.3MB/s ± 2%  +27.31%  (p=0.000 n=9+10)
UnmarshalDataset/canada-32        19.9MB/s ± 2%  21.8MB/s ± 3%   +9.33%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32  13.2MB/s ± 3%  14.7MB/s ± 2%  +11.87%  (p=0.000 n=9+10)
UnmarshalDataset/twitter-32       25.9MB/s ± 3%  31.7MB/s ± 2%  +22.41%  (p=0.000 n=10+9)
UnmarshalDataset/code-32          20.2MB/s ± 5%  22.5MB/s ± 2%  +11.09%  (p=0.000 n=10+9)
UnmarshalDataset/example-32       28.1MB/s ± 4%  33.1MB/s ± 4%  +17.75%  (p=0.000 n=10+10)
UnmarshalSimple/struct-32         10.0MB/s ± 4%  12.6MB/s ± 1%  +26.71%  (p=0.000 n=10+9)
UnmarshalSimple/map-32            7.17MB/s ± 3%  8.73MB/s ± 3%  +21.73%  (p=0.000 n=9+10)
ReferenceFile/struct-32           78.8MB/s ± 5%  93.9MB/s ± 5%  +19.16%  (p=0.000 n=10+10)
ReferenceFile/map-32              47.5MB/s ± 4%  57.0MB/s ± 4%  +19.97%  (p=0.000 n=10+10)
HugoFrontMatter-32                21.6MB/s ± 6%  28.7MB/s ± 3%  +32.81%  (p=0.000 n=9+10)

name                              old alloc/op   new alloc/op   delta
UnmarshalDataset/config-32          7.60MB ± 0%    5.96MB ± 0%  -21.49%  (p=0.000 n=9+9)
UnmarshalDataset/canada-32          84.4MB ± 0%    84.4MB ± 0%   -0.00%  (p=0.000 n=9+10)
UnmarshalDataset/citm_catalog-32    35.7MB ± 0%    35.6MB ± 0%   -0.13%  (p=0.000 n=10+10)
UnmarshalDataset/twitter-32         14.0MB ± 0%    13.5MB ± 0%   -3.48%  (p=0.000 n=10+10)
UnmarshalDataset/code-32            23.2MB ± 0%    22.4MB ± 0%   -3.54%  (p=0.000 n=9+10)
UnmarshalDataset/example-32          202kB ± 0%     193kB ± 0%   -4.41%  (p=0.000 n=10+10)
UnmarshalSimple/struct-32             661B ± 0%      597B ± 0%   -9.68%  (p=0.000 n=10+10)
UnmarshalSimple/map-32              1.04kB ± 0%    0.97kB ± 0%   -6.17%  (p=0.000 n=10+10)
ReferenceFile/struct-32             13.0kB ± 0%    11.6kB ± 0%  -10.84%  (p=0.000 n=10+10)
ReferenceFile/map-32                30.4kB ± 0%    29.0kB ± 0%   -4.63%  (p=0.001 n=8+9)
HugoFrontMatter-32                  8.58kB ± 0%    7.41kB ± 0%  -13.62%  (p=0.002 n=8+10)

name                              old allocs/op  new allocs/op  delta
UnmarshalDataset/config-32            259k ± 0%      233k ± 0%   -9.84%  (p=0.000 n=8+8)
UnmarshalDataset/canada-32            782k ± 0%      782k ± 0%   -0.00%  (p=0.000 n=10+10)
UnmarshalDataset/citm_catalog-32      193k ± 0%      192k ± 0%   -0.38%  (p=0.000 n=8+10)
UnmarshalDataset/twitter-32          62.1k ± 0%     56.9k ± 0%   -8.38%  (p=0.000 n=10+10)
UnmarshalDataset/code-32             1.08M ± 0%     1.06M ± 0%   -1.19%  (p=0.000 n=9+8)
UnmarshalDataset/example-32          1.48k ± 0%     1.36k ± 0%   -8.05%  (p=0.000 n=10+10)
UnmarshalSimple/struct-32             8.00 ± 0%      7.00 ± 0%  -12.50%  (p=0.000 n=10+10)
UnmarshalSimple/map-32                13.0 ± 0%      12.0 ± 0%   -7.69%  (p=0.000 n=10+10)
ReferenceFile/struct-32                204 ± 0%       182 ± 0%  -10.78%  (p=0.000 n=10+10)
ReferenceFile/map-32                   671 ± 0%       649 ± 0%   -3.28%  (p=0.000 n=10+10)
HugoFrontMatter-32                     160 ± 0%       143 ± 0%  -10.62%  (p=0.000 n=10+10)