Align with the api provided by serde_json

[r-marques]

• Re-export to* and from* methods in the root of the crate
• Added a rust-toolchain file to set rustup to nightly by default
• Update README to make it explicit there is a crate published in crates.io

This merge request makes it easier to replace serde_json with serde_json_core when building with no_std.

Example:

#[cfg(not(feature = "std"))]
use serde_json_core as serde_json;

let deserialized = serde_json::from_str("...").unwrap();

[japaric]

Thanks!

bors r+

[bors[bot]]

Build succeeded

• continuous-integration/travis-ci/push