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.