Implement RTS's ghc-debug API

Implement functions that are needed by ghc-debug. These are functions to pause and unpause the RTS and to traverse the heap.

Pausing is needed because otherwise the heap might change during traversal and show an inconsistent state.

(Un-) pausing functions:

• rts_pause()
• rts_unpause()
• rts_isPaused()

Traversal functions:

• rts_listThreads()
• rts_listMiscRoots()

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This ticket is created as a future reference. (We always add ticket numbers to commits.)

There already is a merge request: !1435 (closed)