Change json::to_xxx(&Json) functions to be json::Json::to_xxx(&self)

[brandonson]

to_str, to_pretty_str, to_writer, and to_pretty_writer were at the top
level of extra::json, this moves them into an impl for Json to match
with what's been done for the rest of libextra and libstd. (or at least for vec and str)

Also meant changing some tests.

Closes #8676.

[alexcrichton]

While you're at it, would you mind trying to move away from io::Writer towards rt::io::Writer?

Otherwise this looks good to me, thanks!

[brandonson]

I looked into it, but the new rt::io::extensions::WriterUtil doesn't seem to have a write_str method, nor does it appear to be possible to get a writer which to a string (which io::with_str_writer currently does). Which means I'd essentially have to write at least with_str_writer.

It looks like including something like write_str was discussed in #8089 but then left alone as undecided on whether to include it. I haven't seen anything regarding with_str_writer.

So I guess I need input on whether I should: A) wait for more additions to rt::io supporting strings, or B) write an equivalent to with_str_writer and include it in either json or somewhere in rt::io, then find some way to convert strings into a writable format.

Of course if I'm missing something in rt::io which supports strings, let me know. std::rt isn't included in the docs, unfortunately.

[alexcrichton]

It's true that there aren't as many "nice methods", but I think that it's partially by design that way. You can get write_str with write(str.as_bytes()), and with_str_writer can be done with MemWriter and str::from_owned_bytes.

You can look at the example in the sprintf function inside of std/fmt/mod.rs of how to go from a byte-writer to a string. Ideally there'd be a helper function for doing so in rt::io, but we haven't fully finished moving towards that just yet.

[brson]

Eventually there will be more string support in the new I/O library. Haven't gotten to that part yet.

[brandonson]

So I dove into this today, got a fair bit done, then ran into #8579 and #6551. Using &-ptrs in the Encoder and Decoder forces lifetime constraints where they're used, and in extra::workcache providing those constraints causes ICEs. Those ICEs, in this case, can't be worked around without having unconstrained type errors.

Current status is on this branch: https://github.com/singingboyo/rust/tree/json-to-rt-io if anyone wants to take a look, but it seems to me like rt::io support isn't going to work at the moment unless we want to be using @-ptrs.

[alexcrichton]

Oh well, thanks for investigating!