Make task_rng() return @mut R instead of @@mut R
auto merge of #7216 : kballard/rust/task_rng, r=brson
resolve: limit name suggestions to lev_distance 3

A lev_distance of 5 gives us insane suggestions like:

  error: unresolved name `printl`.  Did you mean `count`?

The lev_distance between "printl" and "count" is 4 here.  So, reduce
that number to 3.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>