Fix emoji decoding error

[wongoo]

What this PR does:
fix emoji decoding error and refactor string decoding algorithm, with 17% performance improvement

Which issue(s) this PR fixes:

Fixes #252 #253

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

- fix emoji decoding error and refactor string decoding algorithm, with 17% performance improvement

[codecov-io]

Codecov Report

Merging #254 (cc4f9a4) into master (4014d7e) will increase coverage by 1.42%.
The diff coverage is 78.04%.

@@            Coverage Diff             @@
##           master     #254      +/-   ##
==========================================
+ Coverage   66.19%   67.61%   +1.42%     
==========================================
  Files          25       25              
  Lines        2801     2671     -130     
==========================================
- Hits         1854     1806      -48     
+ Misses        718      644      -74     
+ Partials      229      221       -8     

Impacted Files	Coverage Δ
string.go	70.70% <78.04%> (+13.99%)	⬆️
decode.go	72.00% <0.00%> (-2.00%)	⬇️

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[wongoo]

benchmark result

before fixing:

BenchmarkDecodeStringAscii
BenchmarkDecodeStringAscii-8                6952            173795 ns/op
BenchmarkDecodeStringUnicode
BenchmarkDecodeStringUnicode-8              5228            229713 ns/op
BenchmarkDecodeStringEmoji
    string_test.go:230: err: bad utf-8 encoding
--- FAIL: BenchmarkDecodeStringEmoji

after fixing:

BenchmarkDecodeStringAscii
BenchmarkDecodeStringAscii-8                7149            145317 ns/op
BenchmarkDecodeStringUnicode
BenchmarkDecodeStringUnicode-8              5768            182145 ns/op
BenchmarkDecodeStringEmoji
BenchmarkDecodeStringEmoji-8                1400            816080 ns/op

with 17% performance improvement.

[fangyincheng]

ci failed
The command "/tmp/tools/license/license-header-checker -v -a -r -i vendor /tmp/tools/license/license.txt . go && [[ -z `git status -s` ]]" exited with 1.