Handle terminal encoding

[fredemmott]

If someone writes to stdout, we currently just send raw bytes. We should respect locale.

While most terminals are either C or some variant of UTF-8, a large variety of encodings are supported, e.g. on recent ubuntu:

fred@homeserver:~# LC_ALL=ja_JP.UTF-8 date
2019年  2月 21日 木曜日 08:52:14 PST
fred@homeserver:~# LC_ALL=ja_JP.EUC_JP date
2019ǯ  2�� 21�� ������ 08:52:22 PST

I suggest:

• defining Hack source files/literals etc as UTF-8 (this needs some wider discussion)
• keeping HSL IO interfaces as raw bytes
• stop using HSL IO interfaces directly here - provide (read|write)(Text|Bytes) that handle locale if appropriate

[fredemmott]

Another question: do any common platforms support UTF-16 terminals, e.g. if we end up supporting windows?

[fredemmott]

Another question: do any common platforms support UTF-16 terminals, e.g. if we end up supporting windows?

Yes, though windows has recently gained UTF-8 support that we might be able to depend on in a few years, depending on what versions we target (if any).