Now we need atomic operations, which are lighter than mutex, more
widely for extension libraries because of Ractor.

This commits adds in documentation to illustrate the different modes you
can start coverage with. Examples are provided to show how to start each
of these modes, along with an explanation of the mode and a description
of the output.

It seems almost no internal codes use RHASH_TBL any longer.  Why not
just eliminate it entirely, so that the macro can be purely ext-only.

Formerly, branch coverage measurement counters are generated for each
compilation traverse of the AST.  However, ensure clause node is
traversed twice; one is for normal-exit case (the resulted bytecode is
embedded in its outer scope), and the other is for exceptional case (the
resulted bytecode is used in catch table).  Two branch coverage counters
are generated for the two cases, but it is not desired.

This changeset revamps the internal representation of branch coverage
measurement.  Branch coverage counters are generated only at the first
visit of a branch node.  Visiting the same node reuses the
already-generated counter, so double counting is avoided.

To fix build failures.