doc: quick pass updating HACKING.txt

This document has rotted a bit since the introduction of
til_module_context_t and til_setup_t types, and til_stream_t.

For now I've just made a fast pass of getting what was already
there to better reflect the current state of things.

There's still no description of til_stream_t, til_tap_t, what
taps/pipes are, etc.  Much of that falls under runtime
configurable settings, and must be added at some point.

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