Parsing speed improvements

[Pikacz]

Checklist

• If applicable, unit tests
• If applicable, create follow-up issues for purchases-android and hybrids

Motivation

I was scrolling through your code and saw

precondition(
    !Thread.isMainThread,
    "Receipt parsing should not run on the main thread."
)

Which suggested that you care about performance of the parser. Since I wanted to improve my profiling skills here we are :)

Issue: #4006

Description

Improved performance of parsing receipt (which is mostly parsing of dates).

From profiler I saw that most of the time is spent in ArraySlice.toDate().
By removing dateString = String and using system Calendar I was able to sigificently speed up the process.

As fallback I left previous ISO8601DateFormatter.default.date

Example results on iPhone 8

	Fastest before changes	Slowest after changes	Improvement
Time measured via Date	0.0146	0.0035	4 times faster
Cycle measured via mach_absolute_time	352095	85862	4 times faster
Profiler counters Cycles	36 953 516	6 863 165	5 times faster
Profiler counters INST_ALL	76 994 013	7 666 052	10 times less
Profiler counters L1D_TLB_MISS	89 366	17 531	5 times less

I've also added some additional test cases based on example receipts I was able to find through internet.

[aboedo]

thanks so much for doing this!

[aboedo]

dumb little nitpicks that you can totally disregard:

• private here is redundant in that the extension is already private, we try not to repeat
  We usually add line breaks after extension definitions and before they end

See examples in the style guide

Also, given that the method already returns an optional type, it feels like the try part is also redundant, I'd maybe go with toDateFastParse

Again, I don't feel particularly strongly about any of these tbh

[Pikacz]

Done.

Please let me know if there is anything else you wish me to change.