Enable use in no_std environments

These changes are fairly invasive to imports and uses of non-libcore types,
but allow for some or none of the freestanding crates (core, rustc_unicode,
alloc, collections) to be supported by serde.

README.md

@@ -718,6 +718,20 @@ Field Annotations:
 | `#[serde(serialize_with="$path")]`      | Call a function `fn<S>(&T, &mut S) -> Result<(), S::Error> where S: Serializer` to serialize this value of type `T` |
 | `#[serde(deserialize_with="$path")]`    | Call a function `fn<D>(&mut D) -> Result<T, D::Error> where D: Deserializer` to deserialize this value of type `T`  |
 
+Using in `no_std` crates
+========================
+
+The core `serde` package defines a number of features to enable usage in a
+variety of freestanding environments. Enable any or none of the following
+features, and use `default-features = false` in your `Cargo.toml`:
+
+- `alloc` (implies `nightly`)
+- `collections` (implies `alloc` and `nightly`)
+- `std` (default)
+
+If you only use `default-features = false`, you will receive a stock `no_std`
+serde with no support for any of the collection types.
+
 Upgrading from Serde 0.6
 ========================