Add no_std support, by adding an std feature (#281)

To make the library work as `no_std` we add an `std` feature which
is on by default. When it is off, we compile as `no_std` and make
parts of the API that require `std::io` conditional on the `std`
feature.

Cargo.toml

@@ -19,6 +19,10 @@ edition = "2018"
 
 publish = false
 
+[features]
+default = ["std"]
+std = []
+
 [dependencies]
 serde = { version = "1.0", optional = true }
 either = { version = "1.5", default-features = false, optional = true }