Move the guts of try_and_catch into a detail namespace
Move the guts of try_and_catch into a detail namespace

Summary:
[Folly] Move the guts of `try_and_catch` into a `detail` namespace.

Just a bit cleaner this way. Also, now we always slice before returning the derived object, so the derived type never escapes.

Reviewed By: luciang

Differential Revision: D4361471

fbshipit-source-id: 5c9567d3c5480ee9943a85139b8f27ba3b9da2d6

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