Check for changes and rewrite only if necessary

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AsyncRewriter currently runs on every build, slowing it down somewhat. It's possible to make it detect changes:

• When generating GeneratedAsync.cs, include a commented list of all source files that contributed to the process. This means files with [RewriteAsync], but also files with functions that were referenced by rewritten functions. Also include a hash of their contents. When rerunning, reread the listed files and check whether the hash changed.
• Include also a list of non-included files. When generating GeneratedAsync.cs, if there's any new file rerun the generation process.