wire: optimize generation of Version Negotiation packets

[marten-seemann]

name                                old time/op    new time/op    delta
ComposeVersionNegotiationPacket-16     900ns ± 2%     317ns ± 3%  -64.74%  (p=0.000 n=10+10)

name                                old alloc/op   new alloc/op   delta
ComposeVersionNegotiationPacket-16      120B ± 0%       64B ± 0%  -46.67%  (p=0.000 n=10+10)

name                                old allocs/op  new allocs/op  delta
ComposeVersionNegotiationPacket-16      6.00 ± 0%      2.00 ± 0%  -66.67%  (p=0.000 n=10+10)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (fbaa941) 84.03% compared to head (9502db6) 84.19%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4278      +/-   ##
==========================================
+ Coverage   84.03%   84.19%   +0.15%     
==========================================
  Files         146      149       +3     
  Lines       15081    15397     +316     
==========================================
+ Hits        12673    12962     +289     
- Misses       1924     1937      +13     
- Partials      484      498      +14     

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