Encode int64 value as string in json codec

[zindel]

Currently, atdgen-runtime encodes int64 value as JSON number. This makes it difficult to use the resulting JSON in the JavaScript environments (node, browser). For example, consider this issue with the Int64.max_int value:

Welcome to Node.js v12.16.2.
Type ".help" for more information.
> JSON.parse("9223372036854775807")
9223372036854776000

Right away, some information is lost. If we encode it as string though, we don't have such problem anymore. Moreover decoding already supports string as the transportation format for int64 (test is added). This change fixes the interoperability with bucklescript ahrefs/melange-atdgen-codec-runtime#22

Nevertheless, the change is breaking, so version bump is needed if merged.

[rr0gi]

change is breaking in the sense that on-the-wire json is changed?

[zindel]

@rr0gi yes. Before 123, after "123"

[Khady]

this needs a mention in CHANGES.md

[AndreyErmilov]

@Khady @zindel You forgot to mention this change in CHANGES.md :)

[mjambon]

Sorry, I made the release (2.3.0) without trying hard to update the changelog. I think it's still a good idea to update it after the fact. You should be able to update CHANGES.md now in git, and edit manually the release page https://github.com/ahrefs/atd/releases/tag/2.3.0 (copy-paste the 2.3.0 section from CHANGES.md to replace what's already there).

[Khady]

Sorry about that. I've updated the file and the release