Use Fprintf instead of Sprintf+WriteString

[nussjustin]

No performance change, but avoids one allocation when (un)subscribing.

name                              old alloc/op   new alloc/op   delta
PublishSpeed-8                       0.00B          0.00B          ~     (all equal)
PubSubSpeed-8                         122B ± 0%      121B ± 0%   -1.14%  (p=0.000 n=5+4)
AsyncSubscriptionCreationSpeed-8    1.02kB ±28%    1.00kB ±28%     ~     (p=0.643 n=5+5)
SyncSubscriptionCreationSpeed-8     66.3kB ± 0%    66.3kB ± 0%   -0.02%  (p=0.008 n=5+5)
InboxCreation-8                      64.0B ± 0%     64.0B ± 0%     ~     (all equal)
Request-8                             693B ± 0%      693B ± 0%     ~     (p=1.000 n=4+5)
OldRequest-8                        2.44kB ± 0%    2.37kB ± 0%   -2.63%  (p=0.029 n=4+4)
PublishGobStruct-8                  14.1kB ± 0%    14.1kB ± 0%     ~     (p=0.921 n=5+5)
JsonMarshalStruct-8                   992B ± 0%      992B ± 0%     ~     (all equal)
PublishJsonStruct-8                 3.78kB ± 0%    3.78kB ± 0%     ~     (p=1.000 n=4+4)
PublishSpeedViaChan-8                 169B ± 0%      169B ± 0%     ~     (all equal)
ProtobufMarshalStruct-8               712B ± 0%      712B ± 0%     ~     (all equal)
PublishProtobufStruct-8             2.70kB ± 1%    2.71kB ± 1%     ~     (p=0.629 n=4+4)

name                              old allocs/op  new allocs/op  delta
PublishSpeed-8                        0.00           0.00          ~     (all equal)
PubSubSpeed-8                         3.00 ± 0%      3.00 ± 0%     ~     (all equal)
AsyncSubscriptionCreationSpeed-8      12.0 ± 0%      11.0 ± 0%   -8.33%  (p=0.029 n=4+4)
SyncSubscriptionCreationSpeed-8       12.0 ± 0%      11.0 ± 0%   -8.33%  (p=0.008 n=5+5)
InboxCreation-8                       2.00 ± 0%      2.00 ± 0%     ~     (all equal)
Request-8                             14.0 ± 0%      14.0 ± 0%     ~     (all equal)
OldRequest-8                          52.0 ± 0%      50.0 ± 0%   -3.85%  (p=0.008 n=5+5)
PublishGobStruct-8                     330 ± 0%       330 ± 0%     ~     (all equal)
JsonMarshalStruct-8                   11.0 ± 0%      11.0 ± 0%     ~     (all equal)
PublishJsonStruct-8                   66.0 ± 0%      66.0 ± 0%     ~     (all equal)
PublishSpeedViaChan-8                 3.00 ± 0%      3.00 ± 0%     ~     (all equal)
ProtobufMarshalStruct-8               14.0 ± 0%      14.0 ± 0%     ~     (all equal)
PublishProtobufStruct-8               66.0 ± 0%      66.0 ± 0%     ~     (all equal)

[coveralls]

Coverage remained the same at 94.512% when pulling fb22414 on nussjustin:fprintf into acf11e4 on nats-io:master.

[tylertreat]

lgtm

[kozlovic]

LGTM