WIP: Add a summary() function for configuration summarization

This has been requested multiple times in the mesa/xorg community, and
apparently in the GStreamer community as well.

This is a very simple function that takes a single dictionary as an
argument, that dictionary may define scalar or container values which
will be printed as summaries. It treats nested dictionaries as another
level of configuration options, which allows grouping options together.

For example:

```meson
sec1 = {'driver' : 'foobar', 'OS' : 'Linux', 'API' : 1.7}
...
sec2 = {'driver' : 'dive comp', 'OS' : 'Minix', 'API' : 1.1.2}

summary({
  'Backend' : 'OpenGL',
  'Server' : sec1,
  'Client' : sec2,
})
```

Which would print something like:

```txt
Configuration Summary:
  Backend = OpenGL
  Server:
    driver = foobar
    OS = Linux
    API = 1.7
  Client:
    driver = dive comp
    OS = Minix
    API = 1.1.2
```

Fixes mesonbuild#757

mesonbuild/interpreter.py

@@ -2012,6 +2012,7 @@ def build_func_dict(self):
                            'subdir': self.func_subdir,
                            'subdir_done': self.func_subdir_done,
                            'subproject': self.func_subproject,
+                           'summary': self.func_summary,
                            'shared_library': self.func_shared_lib,
                            'shared_module': self.func_shared_module,
                            'static_library': self.func_static_lib,
@@ -2606,6 +2607,32 @@ def func_message(self, node, args, kwargs):
         argstr = self.get_message_string_arg(node)
         mlog.log(mlog.bold('Message:'), argstr)
 
+    @FeatureNew('summary', '0.50.0')
+    @noKwargs
+    def func_summary(self, node, args, kwargs):
+        if len(args) != 1:
+            raise InterpreterException('Summary accepts exactly one argument.')
+        arg = args[0]
+        if not isinstance(arg, dict):
+            raise InterpreterException('Summary argument must be a dictionary.')
+
+        seen_dict = False
+        for v in arg.values():
+            if isinstance(v, dict):
+                seen_dict = True
+            elif seen_dict:
+                raise InterpreterException('Summary argument values must be exclusively dictionaries or exclusively not dictionaries.')
+
+        mlog.log('')
+        mlog.log(mlog.bold('Configuration Summary:'))
+        for s, t in arg.items():
+            if isinstance(t, dict):
+                mlog.log(' ', mlog.bold(s + ':'))
+                for k, v in t.items():
+                    mlog.log('   ', k, '=', v)
+            else:
+                mlog.log(' ', s, '=', t)
+
     @FeatureNew('warning', '0.44.0')
     @noKwargs
     def func_warning(self, node, args, kwargs):