Decoder: use lookup table to validate ASCII

[pelletier]

Trade-off: adds 256 bytes to the resulting binary. Go 1.17 stores the table in rodata and inline the call:

        0x0025 00037 (utf8.go:181)      LEAQ    "".invalidAsciiTable(SB), CX
        0x002c 00044 (utf8.go:181)      MOVBLZX (CX)(DX*1), CX
        0x0030 00048 (utf8.go:181)      TESTB   CL, CL

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Benchmark results

(allocations omitted as they are unchanged)

name                        old time/op    new time/op    delta
ParseBasicStringsEasy/1-2     10.6ns ± 0%    10.3ns ± 0%   -2.90%  (p=0.016 n=4+5)
ParseBasicStringsEasy/4-2     16.0ns ± 0%    14.8ns ± 0%   -7.31%  (p=0.008 n=5+5)
ParseBasicStringsEasy/8-2     23.8ns ± 1%    21.0ns ± 1%  -11.94%  (p=0.008 n=5+5)
ParseBasicStringsEasy/16-2    36.0ns ± 1%    32.0ns ± 3%  -11.04%  (p=0.008 n=5+5)
ParseBasicStringsEasy/21-2    44.9ns ± 1%    40.3ns ± 0%  -10.24%  (p=0.016 n=5+4)

name                        old speed      new speed      delta
ParseBasicStringsEasy/1-2    282MB/s ± 0%   290MB/s ± 0%   +2.98%  (p=0.016 n=4+5)
ParseBasicStringsEasy/4-2    375MB/s ± 0%   405MB/s ± 0%   +7.89%  (p=0.008 n=5+5)
ParseBasicStringsEasy/8-2    420MB/s ± 1%   476MB/s ± 1%  +13.53%  (p=0.008 n=5+5)
ParseBasicStringsEasy/16-2   500MB/s ± 1%   562MB/s ± 3%  +12.47%  (p=0.008 n=5+5)
ParseBasicStringsEasy/21-2   512MB/s ± 1%   573MB/s ± 2%  +11.81%  (p=0.008 n=5+5)

name                               old time/op    new time/op    delta
UnmarshalDataset/config-2            24.7ms ± 0%    24.5ms ± 0%  -1.04%  (p=0.008 n=5+5)
UnmarshalDataset/canada-2            84.6ms ± 0%    85.1ms ± 2%    ~     (p=0.413 n=4+5)
UnmarshalDataset/citm_catalog-2      25.8ms ± 2%    25.7ms ± 1%    ~     (p=1.000 n=5+5)
UnmarshalDataset/twitter-2           9.53ms ± 0%    9.38ms ± 0%  -1.51%  (p=0.008 n=5+5)
UnmarshalDataset/code-2               107ms ± 1%     107ms ± 1%    ~     (p=0.222 n=5+5)
UnmarshalDataset/example-2            165µs ± 2%     163µs ± 0%  -1.06%  (p=0.016 n=5+4)
Unmarshal/SimpleDocument/struct-2     574ns ± 1%     570ns ± 1%    ~     (p=0.056 n=5+5)
Unmarshal/SimpleDocument/map-2        848ns ± 1%     845ns ± 0%    ~     (p=0.310 n=5+5)
Unmarshal/ReferenceFile/struct-2     55.8µs ± 0%    54.0µs ± 0%  -3.24%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-2        83.5µs ± 2%    81.0µs ± 0%  -2.98%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-2          14.4µs ± 1%    14.2µs ± 0%  -1.48%  (p=0.008 n=5+5)

name                               old speed      new speed      delta
UnmarshalDataset/config-2          42.4MB/s ± 0%  42.9MB/s ± 0%  +1.05%  (p=0.008 n=5+5)
UnmarshalDataset/canada-2          26.0MB/s ± 0%  25.9MB/s ± 1%    ~     (p=0.460 n=4+5)
UnmarshalDataset/citm_catalog-2    21.6MB/s ± 2%  21.7MB/s ± 1%    ~     (p=0.952 n=5+5)
UnmarshalDataset/twitter-2         46.4MB/s ± 0%  47.1MB/s ± 0%  +1.53%  (p=0.008 n=5+5)
UnmarshalDataset/code-2            25.0MB/s ± 1%  25.1MB/s ± 1%    ~     (p=0.175 n=5+5)
UnmarshalDataset/example-2         49.2MB/s ± 2%  49.7MB/s ± 0%  +1.07%  (p=0.016 n=5+4)
Unmarshal/SimpleDocument/struct-2  19.2MB/s ± 1%  19.3MB/s ± 0%  +0.74%  (p=0.048 n=5+5)
Unmarshal/SimpleDocument/map-2     13.0MB/s ± 1%  13.0MB/s ± 0%    ~     (p=0.254 n=5+5)
Unmarshal/ReferenceFile/struct-2   94.0MB/s ± 0%  97.1MB/s ± 0%  +3.35%  (p=0.008 n=5+5)
Unmarshal/ReferenceFile/map-2      62.8MB/s ± 2%  64.7MB/s ± 0%  +3.06%  (p=0.008 n=5+5)
Unmarshal/HugoFrontMatter-2        37.9MB/s ± 1%  38.4MB/s ± 0%  +1.49%  (p=0.008 n=5+5)