Variadic definitions for `intersection`

[sledorze]

@gcanti I'm not sure you went that path of not but that may be usable to generalise 'intersection' signature: https://github.com/gcanti/io-ts/blob/master/src/index.ts#L1191

I've tried some modifications but it seems to fail with previous typescript versions. Doing a PR in order to iterate/discuss on this.

Branch 312 https://github.com/gcanti/io-ts/tree/312

[gcanti]

@sledorze I'd wait for an official support https://github.com/Microsoft/TypeScript/issues/29594#issuecomment-460761895

[sledorze]

@gcanti thanks for sharing that thread

[clementheliou]

Any news about this issue ? Is there a workaround to use intersection with more than 5 codecs ? Maybe something based on #431 but the type seems a little bit harder to write here...