Reduce dynamic casts

[ChrisBenua]

Description

This pull requests replaces dynamicCasts (as?) with protocol method calls

Motivation

In our company we heavily rely on SnapKit. So my team and I have investigated SnapKit performance in extremely-large apps. We discovered that dynamicCasts have significant performance impact on SnapKit performance: about 5/6 of time taken by Constraint.init (and its call-tree) is occupied by dynamic_cast_existential_1_conditional. All measurements were made by opening the most important and visited screens in our iOS app.

Before optimisations:

After optimisations:

Changes Made

Extended some protocols with new methods that are used to avoid using as?.
Also, we've successfully avoided breaking ABI (at least in our project).

This Pull Request doesn't require to change call site: ConstraintItem.init is internal and I doubt that somebody has class or struct conforming to ConstraintConstantTarget, ConstraintDSL, ConstraintRelatableTarget or ConstraintInsetTarget.

Testing Done

• Compiled for iOS and macOS
• Checked basic layout in Example app
• Checked complex layouts in our app
• Ran unit-tests locally

Please review this pull request and provide feedback. Thanks in advance!