Use WTF-8 as string encoding in binjs file and escape lone surrogate while processing (fixes #190)

[arai-a]

Applied the following changes:

• introduces escaped_wtf8 module, which converts between WTF-8 and UTF-8 + escape sequence (\x7F + XXXX)
• escape the generated JSON in Node side in shift.rs (we could output WTF-8 from Node side and decode/escape WTF-8 in Rust side, but I didn't want to add more module dependency in node side)
  • this means all string is escaped while processing in Rust side
    (but of course no effect if the input has no lone surrogate)
• unescape while writing string to mutlipart file
  • which effectively changed the string encoding of multipart binjs file to WTF-8
  • this needs the change in JS engine side (I have patch, will post shortly)
• escape while reading string from multipart file
• added formatter (escaped_wtf8::for_print) for binjs_dump and used there, in order to see the escaped string in (incorrect but) more similar-to-usual way

[arai-a]

patch for SpiderMonkey side https://bugzilla.mozilla.org/show_bug.cgi?id=1501155

[Yoric]

@SimonSapin has agreed to review this patch. Let's see how I can add him to the mix.

[SimonSapin]

Looks good to me! I didn’t find any bug. I left inline comments about small nits that could make the code a bit more idiomatic or a bit more efficient (though maybe that’s premature optimization). Feel free to ignore any or all of them.