sstables::test_env uses the pimpl idiom, but incompletely. This
prevents reaping some of the benefits.
Complete the pimplification:
- the `impl` nested struct is moved out-of-line
- all non-template member functions are moved out-of-line
- a destructor is declared and defined out-of-line
- the move constructor is also defined (necessary after the destructor is
defined)
After this, we can forward-declare more components.
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