der: add BmpString

[tarcieri]

Adds initial support for a Basic Multilingual Plane (a.k.a. UCS-2) string type.

[baloo]

It looks good, the one change I'd like to introduce would be to avoid silently accepting chars that encode over more than 2 bytes (which this currently does).

I'd prefer that to be a legitimate choice by the consumer of the API if possible (hence the hazmat flag and the new_unchecked naming), something like: 8b6f5f5

[tarcieri]

@baloo I'm not terribly well versed in UCS-2/UTF-16. Is the issue there unpaired surrogates or something different?

Would composing from_utf8 in terms of from_ucs2 catch it, or do we need to check the value of each decoded character (in from_ucs2) to make sure it doesn't exceed u16::MAX?

[carl-wallace]

Looks good to me (no opinion re: encoding to more than two bytes). I dropped it into the work-in-progress P12 code and it worked fine for the (simple) friendly name attributes in that set.

[baloo]

@tarcieri I thought there were issues with the back and forth between str and BmpString but there isn't any. Disregard my previous comment.

[tarcieri]

@baloo I am a little curious if the output will be something other than valid BMP, e.g. codepoints greater than 65535. I suppose I could still check for that?

[tarcieri]

Went ahead and added additional checks in 8d0ed28 e87a10b