Avoid allocating in Conn.NetConn
Avoid allocating in Conn.NetConn

I noticed this in #84. The only (non-stub) implementation of
Conn.NetConn currently wraps the net.Conn in a connLimited
which prevents calls to Read, Write and Close, but also causes
an allocation.

As noted in the issue, this is unnecessary and goes against the
general way of handling these kind of restrictions in the package
which is just documenting what is not allowed instead of trying to
prevent the user from doing stupid things. They will always find a
way anyway, so no need to be too preventive / defensive here.

Since the Conn.NetConn documentation already forbis calling these
methods we can just remove the wrapping altogether and avoid the
allocation.

No new benchmark since this is not an important change and we got
some numbers in #84.

Updates #84

mediocregopher/radix.v3