Mojo: Move mojo/system/embedder to mojo/embedder.

mojo/embedder/README.md

Index: mojo/embedder/README.md
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+Mojo Embedder API
+=================
+
+The Mojo Embedder API is an unstable, internal API to the Mojo system
+implementation. It should be used by code running on top of the system-level
+APIs to set up the Mojo environment (instead of directly instantiating things
+from src/mojo/system).
+
+Example uses: Mojo shell, to set up the Mojo environment for Mojo apps; Chromium
+code, to set up the Mojo IPC system for use between processes. Note that most
+code should use the Mojo Public API (under src/mojo/public) instead. The
+Embedder API should only be used to initialize the environment, set up the
+initial MessagePipe between two processes, etc.