Splits Equinox.Codec into an interfaces library an an impl library, in order to reduce boilerplate at the point of consumption.

The resulting libraries will likely move to be merged with https://github.com/jet/Jet.JsonNet.Converters

Resolves some aspects of #79

Concerns to be resolved before merge:

• should there be a CreateDefault (mirroring the default settings for the serializer) and a Create with different policies which pave for a pit of success
  • with an OptionConverter in by default
  • with a ListConverter there will be no ListConverter, the advice will be to use arrays
  • with NullHandling.Ignore on (i.e. prevent emitting null for null strings and missing options) agreed that sticking with defaults is good
• should Gardelloyd.Custom.Create(encode,tryDecode) become Gardelloyd.Codec.Create(encode,tryDecode) or would the overloading of Codec (and the adjacent Gardelloyd.Core make a mess ??
• Final name: FsCodec