Fix OpCode arbitrary generation w.r.t. non-ASCII chars

Since we generate arbitrary strings and then we send them over a pipe
to Python, we can (and will) generate non-ascii chars, which means
when run in a non-UTF8 locale, this will break with ghc 6.12 and
later. It didn't break on my workstation, since I have a "nice"
locale.

So to fix this, we replace all name generations with explicit
genName/genNameNE and similar, to ensure we only send "nice" strings.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>