Fix serde and decode features without default features

[ascjones]

Previously the following feature combinations caused compilation errors:

cargo check --no-default-features --features serde caused implementation of `Deserialize` is not general enough: because by default PortableForm::String = &'static str

cargo check --no-default-features --features decode failed because parity-scale-codec needs the full feature to support String decoding.

The reason we have these features is to support constructing PortableForm types for encoding in the substrate runtime, which does not support String encoding. See #59 and #35 for more.