ARROW-5108: [Go] implement reading primitive arrays from Arrow file

[sbinet]

This CL adds the generation of flatbuffers-based files for the IPC backend.
This CL adds the basic functionality to read an Arrow file.

[sbinet]

@stuartcarnie@alexandreyc PTAL.

it's a lot of code, but most of it is coming from the automatically generated code from flatbuffers.
one can safely ignore everything under:

• go/arrow/internal/flatbuf
• go/arrow/ipc/feather/fbs

[wesm]

Is there a way to generate the Flatbuffers bindings at build time or do the files have to be checked in?

[codecov-io]

Codecov Report

Merging #4114 into master will decrease coverage by 0.56%.
The diff coverage is 6.17%.

@@ Coverage Diff @@## master #4114 +/- ##
==========================================
- Coverage 88.44% 87.87% -0.57% 
==========================================
Files 752 758 +6 Lines 91515 92163 +648 Branches 1251 1251 ==========================================
+ Hits 80937 80990 +53 - Misses 10457 11052 +595 
Partials 121 121

Impacted Files	Coverage Δ
go/arrow/ipc/message.go	0% <0%> (ø)
go/arrow/ipc/reader.go	0% <0%> (ø)
go/arrow/ipc/ipc.go	0% <0%> (ø)
go/arrow/ipc/file_reader.go	0% <0%> (ø)
go/arrow/ipc/metadata.go	0% <0%> (ø)
go/arrow/internal/bitutil/bitutil.go	93.33% <100%> (+0.11%)	⬆️
go/arrow/ipc/dict.go	100% <100%> (ø)
go/arrow/schema.go	87.77% <35.71%> (-9.6%)	⬇️
js/src/ipc/metadata/json.ts	92.39% <0%> (-4.35%)	⬇️
... and 7 more

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[sbinet]

@wesm: this (unfortunately?) isn't the way Gophers like to do it.
we prefer to have (for security reasons) only Go stdlib-based code being executed as part of the usual go get incantation to retrieve+build code.
and we prefer to have the whole code needed for users, ready be to built (w/o additional build-time tools.)

[sbinet]

the flurry of activity is now done.
I am pretty happy with my refactoring and ready for the writing part (+ support for binary/string types.)

PTAL

[alexandreyc]

Is there any reason why you switched from pointer to value receiver?

[sbinet]

it was mainly to convey that method wouldn't modify the receiver.
I can switch to pointer receiver.

[alexandreyc]

Maybe check for msg.meta and msg.body to be not nil before releasing them?

[sbinet]

I put the check+panic in the NewMessage instead.

[stuartcarnie]

Excited to see this work 🎉

[sbinet]

PTAL.

[nickpoorman]

This is great!