Change FsDataProvider to put data marker attributes in the same file

[sffc]

In order to reduce the number of JSON files, it might make sense for the FsDataProvider to merge marker attributes into the locale files.

So, instead of this:

• foo@1/attr1/en.json: "hello"
• foo@1/attr2/en.json: "world"

we could have foo@1/en.json with the following content:

{
    "attr1": "hello",
    "attr2": "world"
}

CC @Manishearth

[robertbastian]

Or one big file for everything 😃.

I don't really want to make changes to FS-JSON only. This doesn't seem compatible with FS-postcard.

[sffc]

Well I guess the way this works is that FsDataProvider would serialize a ZeroMap<str, DataStruct> which I think should work with postcard.

[sffc]

It does make postcard (and JSON) less efficient since a load operation needs to parse out the whole file. Hmm.

[Manishearth]

I think FS should just support both, selectable at datagen time, leading to .multi.json files (or whatever). We use the multi thing for JSON, where the goal is primarily human readability. We use individual ones for postcard where we actually want the slicing.