Remove allocs from WriteFloat32/WriteFloat64

[brian-brazil]

The use of strconv.FormatFloat causes a string allocation,
by setting aside a reusable buffer and using strconv.AppendFloat
this can be avoided.

Before:
BenchmarkRespond-4 300 5392189 ns/op 618936 B/op 20010 allocs/op

After:
BenchmarkRespond-4 300 4713746 ns/op 139744 B/op 10 allocs/op

This benchmark is using a custom encoder that calls WriteFloat64 20k
times, which is the bulk of the work.

[codecov]

Codecov Report

Merging #234 into master will decrease coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #234      +/-   ##
==========================================
- Coverage    83.5%   83.46%   -0.04%     
==========================================
  Files          44       44              
  Lines        5500     5505       +5     
==========================================
+ Hits         4593     4595       +2     
- Misses        716      718       +2     
- Partials      191      192       +1

Impacted Files	Coverage Δ
feature_stream.go	74.71% <100%> (+0.14%)	⬆️
feature_stream_float.go	100% <100%> (ø)	⬆️
feature_iter_skip.go	75.29% <0%> (-3.53%)	⬇️

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[taowen]

will merge this pr today.

[brian-brazil]

Thanks!