Robust serialisation of IEEE 754 NaN and Inf

[timbod7]

Currently the rule is that ADL types Float and Double are serialised as json numbers. This won't work for NaN and Inf across the language bindings.

A possible alternate rule would be:

ADL types Float and Double are serialized as json numbers, except that NaN values are serialised as the string "NaN", and Inf values are serialised as the string "Inf".