Add optional

[thewilkybarkid]

This adds optional to the Decoder, Encoder and Codec modules, which handles conversion between undefined and Option.

This is a little different to optionFromNullable in that it only works with undefined and not null (optionFromNullable decodes from both, and encodes to null (gcanti/io-ts-types#166)).

Refs #542 (comment).

[thewilkybarkid]

Just tried this with the json Encoder from #638 (as a property in a struct) and found that the undefined is not assignable to Json (i.e. it's not quite the same as not existing in the type).

[thewilkybarkid]

Options I can see are to make the JsonRecord type have undefined properties, or to use something like microsoft/TypeScript#12400 (comment) on the struct types (i.e. to make properties that can have undefined as a value optional).