[prototype] Add type-specific decoder code generator

[tsholmes]

This adds a new objconvgen entrypoint that generates a optimized decoder for a specific type. Since the decoder doesn't need to use reflection and can return all results by-value, it attains a significant speedup from the generic parsing.

Since this is a prototype, I'd like to get some feedback on the approach I took. Any and all suggestions and feedback are welcome. This is a pretty big feature, so I'd like everyone to be on board with it.

Sample benchmark on a structure with strings, times, and nested arrays

BenchmarkDecodeCBOR-8                  200000          6431 ns/op          80 B/op           6 allocs/op
BenchmarkDecodeJSON-8                  200000          8607 ns/op          80 B/op           6 allocs/op
BenchmarkDecodeCBORGenerated-8         500000          3319 ns/op          48 B/op           5 allocs/op
BenchmarkDecodeJSONGenerated-8         300000          5425 ns/op          48 B/op           5 allocs/op

What's supported right now:

• structures
• arrays (not slices)
• primitives
• time.Time
• aliased types
• types in different packages

What's not supported right now:

• slices
• maps
• time.Duration
• []byte/[N]byte special case

What definitely needs to be done before this could be merged:

• support json and objconv struct tags
• write the output to a file and goimports/gofmt it
• add tests on some generated code
• add benchmarks on some generated code

What would be nice to clean up before this is merged:

• remove dependence of generated code on github.com/segmentio/objconv/generate/util package

[achille-roussel]

Looking great overall 👍

One last comment, what do you think about using a sub-command of objconv for the CLI? (objconv generate)?

I can see how a syntax like this could be convenient as well:

objconv generate github.com/segmentio/pkg.Type

It can output to stdout by default, then keep the overwrite with the -o option you have right now.

[achille-roussel]

Should we check that the fractional part is zero as well?

This could help => https://golang.org/pkg/math/#Modf

[tsholmes]

Ah that's what the double cast was for: float64(int64(fv)) != fv. this also catches very large numbers that get clobbered on a cast to int64

[achille-roussel]

How about naming the package genutil? Seems like it could communicate a little better what it's intended to be used for?

Could be a good idea to add a doc.go with a package documentation stating that the functions are here to support generated code and should not be accessed directly.

[achille-roussel]

Is using a _ required here? golint usually complains about that.

[achille-roussel]

How about having a signature based on io.Writer? Maybe something like this:

func WriteDecoder(w io.Writer, path, typ string) error