Fix #782, #1483, #1649: Unicode GHCi input

Fix #782, #1483, #1649: Unicode GHCi input
GHCi input is now treated universally as UTF-8, except for the Windows
console where we do the correct conversion from the current code
page (see System.Win32.stringToUnicode).

That leaves non-UTF-8 locales on Unix as unsupported, but (a) we only
accept source files in UTF-8 anyway, and (b) UTF-8 is quite ubiquitous
as the default locale.