!perf: Use string for iteration and error reporting

[romshark]

The string type is smaller than a []byte and can be copied much faster. Copying structs <= 32 bytes on modern 64-bit systems is essentially free whereas Error[[]byte] is 40 bytes wide
making jscan work faster with string than with []byte. This PR eliminates the overhead at the cost of backward compatibility improving performance by about 14%.

• 👎🏻 This change requires the use of unsafe.
• 👎🏻 Requires backward incompatible changes to type Error:
  • Removed type parameter.
  • Removed field Src.
• 👍🏻 Slightly better performance overall.
• 👍🏻 Type Error is no longer required to be type-parameterized.

An alternative solution is presented in #25.

Benchmark results:

goos: darwin
goarch: arm64
pkg: github.com/romshark/jscan/v2
                                    │  main.bench  │        string-iterator.bench        │
                                    │    sec/op    │   sec/op     vs base                │
CalcStats/miniscule_1b__________-10    20.63n ± 0%   14.28n ± 0%  -30.80% (p=0.000 n=10)
CalcStats/tiny_8b_______________-10    27.84n ± 0%   25.92n ± 0%   -6.90% (p=0.000 n=10)
CalcStats/small_336b____________-10    311.8n ± 0%   317.2n ± 0%   +1.75% (p=0.000 n=10)
CalcStats/large_26m_____________-10    13.84m ± 0%   13.79m ± 0%   -0.35% (p=0.000 n=10)
CalcStats/nasa_SxSW_2016_125k___-10    115.4µ ± 0%   114.2µ ± 0%   -1.09% (p=0.000 n=10)
CalcStats/escaped_3k____________-10    1.399µ ± 0%   1.397µ ± 0%   -0.14% (p=0.000 n=10)
CalcStats/array_int_1024_12k____-10    13.83µ ± 0%   13.31µ ± 0%   -3.72% (p=0.000 n=10)
CalcStats/array_dec_1024_10k____-10    11.73µ ± 4%   11.91µ ± 7%        ~ (p=0.105 n=10)
CalcStats/array_nullbool_1024_5k-10    7.482µ ± 1%   7.480µ ± 1%        ~ (p=0.436 n=10)
CalcStats/array_str_1024_639k___-10    146.4µ ± 0%   146.2µ ± 0%   -0.15% (p=0.000 n=10)
Valid/deeparray_____________-10       15.680n ± 1%   4.966n ± 0%  -68.33% (p=0.000 n=10)
Valid/unwind_stack__________-10       15.665n ± 1%   4.966n ± 0%  -68.30% (p=0.000 n=10)
Valid/miniscule_1b__________-10       11.430n ± 0%   7.760n ± 0%  -32.11% (p=0.000 n=10)
Valid/tiny_8b_______________-10        17.38n ± 0%   16.76n ± 0%   -3.57% (p=0.000 n=10)
Valid/small_336b____________-10        236.2n ± 0%   225.9n ± 0%   -4.34% (p=0.000 n=10)
Valid/large_26m_____________-10        11.09m ± 0%   11.00m ± 0%   -0.80% (p=0.000 n=10)
Valid/nasa_SxSW_2016_125k___-10        85.25µ ± 0%   83.16µ ± 0%   -2.45% (p=0.000 n=10)
Valid/escaped_3k____________-10        1.387µ ± 0%   1.386µ ± 0%   -0.07% (p=0.001 n=10)
Valid/array_int_1024_12k____-10        9.135µ ± 1%   9.447µ ± 0%   +3.42% (p=0.000 n=10)
Valid/array_dec_1024_10k____-10        8.672µ ± 0%   8.661µ ± 0%        ~ (p=0.280 n=10)
Valid/array_nullbool_1024_5k-10        3.415µ ± 5%   3.243µ ± 4%   -5.04% (p=0.001 n=10)
Valid/array_str_1024_639k___-10        142.3µ ± 0%   142.4µ ± 0%        ~ (p=0.105 n=10)
Yo/bytes-10                                          13.04n ± 0%
Yo/string-10                                         12.89n ± 1%
geomean                                3.038µ        1.681µ       -13.86%

[romshark]

Alternatively, the error reporting can be changed to avoid needing Src in the first place.
The downside would be slightly less convenient error messages:

error at index 6: illegal control character

instead of:

error at index 6 (0x02): illegal control character

But this can still be replicated by the API user if necessary.