Add the option replace RFC

text/0000-option-replace.md

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+- Feature Name: `option-replace`
+- Start Date: 2017-01-16
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+The standard library provides the `Option::take` method that returns the optional inner value and leaves a `None` in place. This RFC proposes the addition of `Option::replace` to complete the previously cited method, this method replaces the actual value in the option by the value given in parameter, returning the old value if present, leaving a `Some` in its place without deinitializing either one.
+
+# Motivation
+[motivation]: #motivation
+
+Why are we doing this? What use cases does it support? What is the expected outcome?
+
+How do you replace a value inside an `Option`, you can use `mem::replace` but it can be really unconvenient to import the `mem` module just for that. Why not adding a useful method to do that ?
+This is the same kind of method than the `take` one.
+
+# Detailed design
+[design]: #detailed-design
+
+This methid will be added to the `core::option::Option` type implementation:
+
+```rust
+use core::mem::replace;
+
+pub fn replace(&mut self, value: T) -> Option<T> {
+    mem::replace(self, Some(value))
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+Use directly `mem::replace`.
+
+# Alternatives
+[alternatives]: #alternatives
+
+None.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None.