Bk/performance optimizations

[bryankeller]

Details

This is the first of three PRs to optimize MagazineLayout. I did extensive profiling, and found several hot spots.

For a collection view with 100,000 items in it, after this wave of optimizations, the time in prepare has been greatly reduced.

• prepare on first load: 60ms -> 4.5ms (13x reduction)
• prepare after invalidating all delegate layout values: 58ms -> 5ms (11x reduction)
• prepare after rotating the device: 58ms -> 8μs (725x reduction lol)

The changes in this PR don't get us all the way to those numbers, but after all three are merged, we'll get the full benefits.

These are the optimizations:

• Reuse a single IndexPath instance in hot paths
• Avoid protocol conformance checks on every delegate access
• Avoid relatively-expensive UIKit calls when reading the current modelState
• Defer calculating frames in each section
• Greatly reduces the amount of work when just updating the width of the collection view to something new

I'd suggest reviewing commit-by-commit - that should make it more clear what changes are related to certain optimizations.

How Has This Been Tested

Tested in demo app and Airbnb app. Additionally, all unit tests pass.

Types of changes

• Docs change / refactoring / dependency upgrade
• Perf fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

[bryankeller]

Fixed, thanks @brynbodayle !