Fixing #184 out of memory

[rusq]

Fixes #184

• benchmark and fixture generator
• memoise message time
• isolate init from the benchmark
• optimised slicing
• use slice of pointers, instead of slice of exportMessages
• address by index, save by pointer

Running a benchmark for comparison:

go test -benchmem -run=^$ -bench ^BenchmarkByDate$ github.com/rusq/slackdump/v2/export

Test rig:

goos: darwin
goarch: amd64
pkg: github.com/rusq/slackdump/v2/export
cpu: Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz

Benchmark stats for 10,000 generated messages:

* Initial *
BenchmarkByDate-16    	      16	  67085111 ns/op	52772423 B/op	  368681 allocs/op

* Intermediate *
BenchmarkByDate-16            54          22102931 ns/op        43947099 B/op      30025 allocs/op

* Final *
BenchmarkByDate-16           258           4612715 ns/op         1638080 B/op      20023 allocs/op

For million messages:

* Intermediate *
BenchmarkByDate-16             1        4210968611 ns/op        4777708088 B/op  3000049 allocs/op

* Final *
BenchmarkByDate-16             2         813553898 ns/op        169678572 B/op   2000043 allocs/op

Summary:

• 14.54 times faster
• 19.7 times less Bytes allocated per operation
• 17.7 times less allocations per operation