Add support for embedding pretty printers via the #[debugger_visualizer] attribute. Add tests for embedding pretty printers and update documentation.

Ensure all error checking for `#[debugger_visualizer]` is done up front and not when the `debugger_visualizer` query is run.

Clean up potential ODR violations when embedding pretty printers into the `__rustc_debug_gdb_scripts_section__` section.

Respond to PR comments and update documentation.

src/test/debuginfo/embedded-visualizer.py

@@ -0,0 +1,23 @@
+import gdb
+
+class LinePrinter:
+    "Print a Line"
+
+    def __init__(self, val):
+        self.val = val
+        self.a = val["a"]
+        self.b = val["b"]
+
+    def to_string(self):
+        return "({}, {})".format(self.a, self.b)
+
+def lookup(val):
+    lookup_tag = val.type.tag
+    if lookup_tag is None:
+        return None
+    if "embedded_visualizer::Line" == lookup_tag:
+        return LinePrinter(val)
+
+    return None
+
+gdb.current_objfile().pretty_printers.append(lookup)