Move compiler-support code in object.d to the bottom of the file with the others

[JinShil]

Followup to #2647, #2644, #2643, #2634, #2763, #2765, #2766, #2769, #2772 , #2774

This is a continuation of work to clean up object.d.

This PR just moves the remaining individual compiler-support features to the bottom of the file to de-emphasize them and keep them separate from the code accessible to users.

[dlang-bot]

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Testing this PR locally

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dub fetch digger
dub run digger -- build "master + druntime#2775"