The only remaining differences between vfprintf.cpp and vfwprintf.cpp
after this are the wide/narrow conversions for %c, %m, and %s. I've used
"chars" and "bytes" for the named constants for the directions because
(a) I find -1 and 1 pretty confusing and (b) although "narrow" is the
obvious opposite of "wide", only Windows actually moved to wide
characters, so "narrow" (aka "multibyte", and probably "utf8") is the
default/normal case. Even though C confuses bytes and characters via its
`char` type, "bytes" versus "chars" seems like the appropriate
terminology (and it's what Java/Python use).
Also improve the swprintf tests assertion so failures are readable.
Test: treehugger
Change-Id: Ife8f70f65ec28d96058a7d68df353945524835d2
531199c56c