Lots of work on improving debugging.

doc macro can take no arguments and print out
all bindings. Fix an issues with the vm skipping
over a breakpoint in some situations.

Add examples/debugger.janet for proof of concept
debugger.

examples/debug.janet

@@ -0,0 +1,8 @@
+# Load this file and run (myfn) to see the debugger
+
+(defn myfn
+  []
+  (debug)
+  (for i 0 10 (print i)))
+
+(debug/fbreak myfn 3)