SWEEP: Make private/internal fields readonly where possible, #661

[paulirwin]

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Makes private/internal fields readonly where possible.

Fixes #661

Description

This PR used ReSharper/Rider's analyzer to find fields that could be made readonly. This PR does not affect any static fields (that's #1079) or protected fields (which might be modified by subclasses). In a few cases, instance fields were made const or static readonly where possible.

[NightOwl888]

Thanks for the PR. This looks good to merge.