Remove serialization support in DynamicProxy

[stakx]

I am making this a draft to prevent accidental merging (although it's mostly ready); after all, we haven't decided yet whether to keep or retire serialization support in DynamicProxy. Until that time, consider this an "exploration" to help us better understand serialization in DynamicProxy.

This PR is perhaps best reviewed commit by commit, for the following reason:

#501 (comment) gave a summary of the different usages of FEATURE_SERIALIZATION in DynamicProxy, and appears to have been largely correct. I used it as a guideline to "undo" serialization from DynamicProxy, in the following order:

1. Make all exception types serializable across all targets. (Update: Moved into a separate PR Make all exception types serializable #515.)

2. Change DynamicProxy so generated proxy and invocation types are no longer serializable. (Those two kinds of generated types cannot be easily done piecemeal because they share a common base class, BaseProxyGenerator. Incorrect: Invocation type generators ultimately subtype IGenerator<T>, not BaseProxyGenerator.)

3. Finally, abandon LoadAssemblyIntoCache and get rid of the requirement that "supporting types" (hooks, interceptors, interceptor selectors, mixins) should be made serializable.

The net result is that DynamicProxy code no longer makes use of FEATURE_SERIALIZATION —the remaining usages are inside DictionaryAdapter and the logging facility.

Closes #367.

[jonorossi]

I won't get time to review this PR today, quite a lot to look at.

Did you want to submit your first bullet point to clean up exceptions (and maybe one other commit to fix a ifdef) as a separate pull request so we can get those in.

[stakx]

I did as you suggested and moved some things into separate PRs. The one for the incorrect compilation symbol is already merged, it was trivial and you've got enough boring conditional symbol obliteration PRs to review as it is. 😆

[stakx]

Let's close this... at least for the time being. If we really do want to remove serialization support, we can always reactivate this PR.