feat: lazy initialization codec

[xieyuschen]

What I have changed

I have applied the TODO: make this lazy initialization instead left by @sagikazarmark . The main changes are listed as following:

• lazily initialize the codec
• merge encoder and decoder to codec.
  This change is caused as I try to create a new codec by calling a method of a nil pointer with type. I want the decoder and encoder to share the same object constructed by the method.
• create constructor.Codec interface to avoid cycle import.

Benchmark

I use this command to run the benchmark across commits:

git checkout 214f242 
go test -v -bench=. -count=10 > old.out
git checkout 60d2632
go test -v -bench=. -count=10  > new.out
benchstat old.out new.out

The output is:

goos: darwin
goarch: amd64
pkg: github.com/spf13/viper
cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
                    │    old.out    │               new.out               │
                    │    sec/op     │   sec/op     vs base                │
New-12                2972.5n ± 77%   431.8n ± 3%  -85.48% (p=0.000 n=10)
_unmarshalReader-12    148.4µ ±  7%   139.1µ ± 3%   -6.25% (p=0.019 n=10)
GetBool-12             186.6n ± 18%   180.6n ± 2%   -3.22% (p=0.017 n=10)
Get-12                 157.5n ±  7%   156.6n ± 1%        ~ (p=0.436 n=10)
GetBoolFromMap-12      3.944n ± 28%   3.878n ± 2%        ~ (p=0.052 n=10)
geomean                551.7n         366.2n       -33.62%

For me, I think this change doesn't reduce the efficiency of viper.

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[github-actions]

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[sagikazarmark]

Wow, thanks @xieyuschen !

Give me some time to review this PR, but I really appreciate the contribution.

One note off the bat: performance generally is not a huge concern in case of Viper as it's mostly initialized/used at the beginning of an application run, so readability is generally preferred over rigorous performance optimization.

[xieyuschen]

Wow, thanks @xieyuschen !

Give me some time to review this PR, but I really appreciate the contribution.

One note off the bat: performance generally is not a huge concern in case of Viper as it's mostly initialized/used at the beginning of an application run, so readability is generally preferred over rigorous performance optimization.

I agree with you, and I have changed the implementation in a go style. @sagikazarmark Could you kindly check it if you have time?

Regards