Use type for omit output if that's the struct's input

[IgnusG]

This change basically allows a type struct as input to omit to be preserved. It's not very useful for actual checking since type accepts anything (even the "omitted" key) but it does remove the key from the returned type and preserves the intuitive behavior behind type.

I believe this relates to #829 but it's missing implementations for pick and partial.

[ianstormtaylor]

Thanks for this @IgnusG! If you'd be down to write a PR for pick or partial I'd be happy to merge those as well.

[jon-certik]

Hi @ianstormtaylor, pick and partial have been fixed by PR #1149 and #1150. Would really appreciate it if these patches can be merged soon! Otherwise the current loopholes significantly undermine the usability of the library on complex types.