encoding/json: implement type override for serialization

The fork adds the following APIs:

	func (*Encoder) Register(f interface{})
	func (*Decoder) Register(f interface{})

where f is a function that determines serialization for a specific type and takes precedence over the MarshalJSON/UnmarshalJSON methods.

Name		Name	Last commit message	Last commit date
Latest commit History 2 Commits
.idea		.idea
testdata		testdata
LICENSE		LICENSE
PATENTS		PATENTS
README.md		README.md
bench_test.go		bench_test.go
decode.go		decode.go
decode_test.go		decode_test.go
encode.go		encode.go
encode_test.go		encode_test.go
example_marshaling_test.go		example_marshaling_test.go
example_test.go		example_test.go
example_text_marshaling_test.go		example_text_marshaling_test.go
fold.go		fold.go
fold_test.go		fold_test.go
fuzz.go		fuzz.go
go.mod		go.mod
indent.go		indent.go
number_test.go		number_test.go
scanner.go		scanner.go
scanner_test.go		scanner_test.go
stream.go		stream.go
stream_test.go		stream_test.go
tables.go		tables.go
tagkey_test.go		tagkey_test.go
tags.go		tags.go
tags_test.go		tags_test.go

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