make next command less skipping

[bicycle1885]

This PR makes the next command less skipping. The motivation is described here: https://github.com/JuliaDebug/Debugger.jl/issues/291.

~~My approach is to add a keyword argument (sameline) to the maybe_next_call! function so that it tires to find a call or a return statement without stepping forward to the next lines. If it fails, it stops at the current program counter.~~

With this patch (and a minor modification to Debugger.jl), assignments and branches are no longer skipped and now it works like debuggers of other languages (such as Python's ipdb):

julia> function foo(x)
           #println(x)
           y = 0
           if x
               y = 1
           end
           return y
       end
foo (generic function with 1 method)

julia> @enter foo(true)
In foo(x) at REPL[1]:1
 1  function foo(x)
 2      #println(x)
>3      y = 0
 4      if x
 5          y = 1
 6      end
 7      return y
 8  end

About to run: 0
1|debug> n
In foo(x) at REPL[1]:1
 1  function foo(x)
 2      #println(x)
 3      y = 0
>4      if x
 5          y = 1
 6      end
 7      return y
 8  end

About to run: goto %4 if not _J2
1|debug>
In foo(x) at REPL[1]:1
 1  function foo(x)
 2      #println(x)
 3      y = 0
 4      if x
>5          y = 1
 6      end
 7      return y
 8  end

About to run: 1
1|debug>
In foo(x) at REPL[1]:1
 3      y = 0
 4      if x
 5          y = 1
 6      end
>7      return y
 8  end

About to run: return 1
1|debug>
1

[KristofferC]

About to run: goto %4 if not _J2

I don't think it is good to stop at these internal looking statements.

[pfitzseb]

Although that seems to be more of a printing issue? Stopping at branch conditions makes sense to me.

[bicycle1885]

How to print assignments and branch conditions is a separated problem, and it should be fixed in Debugger.jl.

NOTE: I found the current impl is completely broken. So, there is no need to check the patch at the moment.

[bicycle1885]

I've changed the design: next_line! continues to step forward until it finds a statement which is one of:

• function calls
• assignments (both local and global)
• returns
• goto nodes (both conditional and unconditional)

Here, "assignment" excludes assignments to compiler-generated variables. I'm not perfectly sure how it can be achieved, but it now checks if slotnames corresponding to the variable is Symbol("") or not. Any better ideas are welcomed.