Improve json decoding

[cmaglie]

I found that the standard golang JSON decoder is very slow. This is particularly noticeable because the library_index.json is constantly growing (currently we are at ~20Mb) and the parsing time adds up at every invocation of the cli.

This PR:

• use easyjson library for parsing the library_index.json
• updates the testdata/library_index.json with the latest current
• add a benchmark to test JSON decoding performances

Here the result:

$ go test -v -benchmem -bench BenchmarkIndexParsing github.com/arduino/arduino-cli/arduino/libraries/librariesindex
=== RUN   TestIndexer
--- PASS: TestIndexer (0.16s)
goos: linux
goarch: amd64
pkg: github.com/arduino/arduino-cli/arduino/libraries/librariesindex
cpu: Intel(R) Core(TM) i7-8550U CPU @ 1.80GHz
BenchmarkIndexParsingStdJSON
BenchmarkIndexParsingStdJSON-8                 5         214872730 ns/op          94.52 MB/s    58956539 B/op     418973 allocs/op
BenchmarkIndexParsingEasyJSON
BenchmarkIndexParsingEasyJSON-8               16          69215472 ns/op         293.42 MB/s    56162664 B/op     418966 allocs/op
PASS
ok      github.com/arduino/arduino-cli/arduino/libraries/librariesindex 4.442s

[per1234]

Would it be possible to set up some infrastructure for the code generation?

• Task that runs the generation command
• GitHub Actions workflow that runs the task and then checks for a diff (which would indicate that the generated code has gone out of sync)

I did that for the generated code in Arduino Lint and found it to be very useful. The benefits should be even more here where there are more contributors.

Probably should also be documented in the contributor guide:
https://github.com/arduino/arduino-cli/blob/master/docs/CONTRIBUTING.md

[ubidefeo]

I have run the build from this PR for about one hour on several projects and have not been able to break it.
From a usage point of view it LGTM

[cmaglie]

For the record: we are using a patched version of easyjson until this patch is accepted upstream: mailru/easyjson#372