We provide here an embedded interactive debugger to let the user control
and inspect its program at symbolic level, with the same features and
commands as classical debuggers: gdb, lldb or delve.

The debugger is enabled by setting the `-debug` flag in `gno run`
command, which loads the target program and immediately shows a debugger
prompt on the console:

	$ gno run -debug /tmp/my-program.gno
	Welcome to the Gnovm debugger. Type 'help' for list of commands.
	dbg>

Providing `-debug-addr` flag allows to start a remote debugging session,
and not interfer with the program stdin and stdout.
For example, in a first terminal:

	$ gno run -debug-addr :4000 /tmp/my-program.gno
	Waiting for debugger client to connect at :4000

And in a second terminal, using a netcat like nc(1):

	$ nc localhost 4000
	Welcome to the Gnovm debugger. Type 'help' for list of commands.
	dbg>

The debugger works by intercepting each execution step at virtual
machine level (each iteration within `Machine.Run` loop) to a
callback, which in turns can provide a debugger command REPL or
check if the execution can proceed to the next step, etc.

This change request is still work-in-progress, as many debugger commands
are not ready yet. Nevertheless, the general logic and structure is
there. It is possible to `continue`, `stepi`, `detach`, etc and get
a general feedback of the user experience and the impact on the code.

Efforts are made to make this feature minimally intrusive in the actual
VM, and not interfering when the debugger is not used.

It is planned shortly after this PR is integrated to add the capacity
to attach to an already running program, and to taylor the data format
for existing debugging environments such as VScode, etc, as demand
arises.

debugEvalExpr evaluates a Go expression in the context of the VM and returns
the corresponding value, or an error.
The supported expression syntax is a small subset of Go expressions:
basic literals, identifiers, selectors, index expressions, or a combination
of those are supported, but none of function calls, arithmetic, logic or
assign operations, type assertions of convertions.

Co-authored-by: Morgan <git@howl.moe>

This reverts commit 7455761.

Fix also list for sources comming from memory packages.

The connection is now initiated of the debugger state machine, so
the error can be processed from the start. It also allows slight
simplifcations.

Also improve test coverage (now > 96% for debugger.go).