Reduce slightly allocations when reading/writing HTTP messages

[nickrobinson251]

1. write direct to io rather than first creating strings
   • we need to be careful to string any numbers before doing this, but doing this selectively still reduces allocations
2. don't create a VersionNumber every time we parse a message (in parse_status_line!/parse_request_line!), as the VersionNumber constructor relies on regex matching which allocates, instead we can define out own much simpler type with allocation-free parsing

Together this takes a simple GET request from 170 -> 155 allocations

[codecov-commenter]

Codecov Report

Merging #950 (2fe988b) into master (7a54ffc) will decrease coverage by 0.62%. The diff coverage is 85.93%.

:exclamation: Current head 2fe988b differs from pull request most recent head d59f8ae. Consider uploading reports for the commit d59f8ae to get more accurate results

@@            Coverage Diff             @@
##           master     #950      +/-   ##
==========================================
- Coverage   79.85%   79.23%   -0.63%     
==========================================
  Files          36       36              
  Lines        2969     3010      +41     
==========================================
+ Hits         2371     2385      +14     
- Misses        598      625      +27     

Impacted Files	Coverage Δ
src/ConnectionPool.jl	85.40% <ø> (-0.86%)	:arrow_down:
src/Servers.jl	79.47% <0.00%> (ø)
src/Strings.jl	90.24% <79.48%> (-9.76%)	:arrow_down:
src/Messages.jl	86.04% <100.00%> (+0.08%)	:arrow_up:
src/Parsers.jl	97.29% <100.00%> (ø)
src/Streams.jl	93.75% <100.00%> (-1.14%)	:arrow_down:
src/clientlayers/MessageRequest.jl	92.30% <100.00%> (ø)
src/access_log.jl	80.00% <0.00%> (-13.34%)	:arrow_down:
src/Exceptions.jl	91.89% <0.00%> (-2.71%)	:arrow_down:
src/connectionpools.jl	70.70% <0.00%> (-2.03%)	:arrow_down:
... and 6 more

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[nickrobinson251]

tests pass locally. not sure if we want to resolve #951 and get a clean CI run before merging here?